diff --git a/Project.toml b/Project.toml
index 97bd214..6a105b7 100644
--- a/Project.toml
+++ b/Project.toml
@@ -21,6 +21,7 @@ StatsBase = "2913bbd2-ae8a-5f71-8c99-4fb6c76f3a91"
 [compat]
 DataStructures = "0.18.20"
 GaussianDistributions = "0.5.2"
+SSMProblems = "0.4.0"
 StatsBase = "0.34.3"
 
 [extras]
diff --git a/src/GeneralisedFilters.jl b/src/GeneralisedFilters.jl
index da9b8d2..d53589f 100644
--- a/src/GeneralisedFilters.jl
+++ b/src/GeneralisedFilters.jl
@@ -13,6 +13,8 @@ using NNlib
 
 abstract type AbstractFilter <: AbstractSampler end
 
+abstract type AbstractParticleFilter{N} <: AbstractFilter end
+
 """
     predict([rng,] model, alg, iter, state; kwargs...)
 
@@ -41,6 +43,22 @@ Perform a combined predict and update call on a single iteration of the filter.
 """
 function step end
 
+"""
+    reset_weights!(log_weights, filter)
+
+Reset container log-weights after a resampling step
+"""
+function reset_weights! end
+
+"""
+    update_weights!
+"""
+function update_weights! end
+
+function log_marginal end
+
+function update_ref! end
+
 function initialise(model, alg; kwargs...)
     return initialise(default_rng(), model, alg; kwargs...)
 end
@@ -106,6 +124,7 @@ include("models/hierarchical.jl")
 
 # Filtering/smoothing algorithms
 include("algorithms/bootstrap.jl")
+include("algorithms/apf.jl")
 include("algorithms/kalman.jl")
 include("algorithms/forward.jl")
 include("algorithms/rbpf.jl")
diff --git a/src/algorithms/apf.jl b/src/algorithms/apf.jl
new file mode 100644
index 0000000..f4a9df6
--- /dev/null
+++ b/src/algorithms/apf.jl
@@ -0,0 +1,116 @@
+export AuxiliaryParticleFilter, APF
+
+mutable struct AuxiliaryParticleFilter{N,RS<:AbstractConditionalResampler} <: AbstractParticleFilter{N}
+    resampler::RS
+    aux::Vector # Auxiliary weights
+end
+
+function AuxiliaryParticleFilter(
+    N::Integer; threshold::Real=0., resampler::AbstractResampler=Systematic()
+)
+    conditional_resampler = ESSResampler(threshold, resampler)
+    return AuxiliaryParticleFilter{N,typeof(conditional_resampler)}(conditional_resampler, zeros(N))
+end
+
+const APF = AuxiliaryParticleFilter
+
+function initialise(
+    rng::AbstractRNG,
+    model::StateSpaceModel{T},
+    filter::AuxiliaryParticleFilter{N},
+    ref_state::Union{Nothing,AbstractVector}=nothing,
+    kwargs...,
+) where {N,T}
+    initial_states = map(x -> SSMProblems.simulate(rng, model.dyn; kwargs...), 1:N)
+    initial_weights = zeros(T, N)
+
+    return update_ref!(ParticleContainer(initial_states, initial_weights), ref_state, filter)
+end
+
+function update_weights!(
+    rng::AbstractRNG, filter, model, step, states, observation; kwargs...
+)
+    simulation_weights = eta(rng, model, step, states, observation)
+    return states.log_weights += simulation_weights
+end
+
+function predict(
+    rng::AbstractRNG,
+    model::StateSpaceModel,
+    filter::AuxiliaryParticleFilter,
+    step::Integer,
+    states::ParticleContainer{T},
+    observation;
+    ref_state::Union{Nothing,AbstractVector{T}}=nothing,
+    kwargs...,
+) where {T}
+    # states = update_weights!(rng, filter.eta, model, step, states.filtered, observation; kwargs...)
+
+    # Compute auxilary weights
+    # POC: use the simplest approximation to the predictive likelihood
+    # Ideally should be something like update_weights!(filter, ...)
+    predicted = map(
+        x -> mean(SSMProblems.distribution(model.dyn, step, x; kwargs...)),
+        states.filtered.particles,
+    )
+    auxiliary_weights = map(
+        x -> SSMProblems.logdensity(model.obs, step, x, observation; kwargs...), predicted
+    )
+    states.filtered.log_weights .+= auxiliary_weights
+    filter.aux = auxiliary_weights
+
+    states.proposed, states.ancestors = resample(rng, filter.resampler, states.filtered, filter)
+    states.proposed.particles = map(
+        x -> SSMProblems.simulate(rng, model.dyn, step, x; kwargs...),
+        states.proposed.particles,
+    )
+
+    return update_ref!(states, ref_state, filter, step)
+end
+
+function update(
+    model::StateSpaceModel{T},
+    filter::AuxiliaryParticleFilter,
+    step::Integer,
+    states::ParticleContainer,
+    observation;
+    kwargs...,
+) where {T}
+    @debug "step $step"
+    log_increments = map(
+        x -> SSMProblems.logdensity(model.obs, step, x, observation; kwargs...),
+        collect(states.proposed.particles),
+    )
+
+    states.filtered.log_weights = states.proposed.log_weights + log_increments
+    states.filtered.particles = states.proposed.particles
+
+    return states, logmarginal(states, filter)
+end
+
+function step(
+    rng::AbstractRNG,
+    model::AbstractStateSpaceModel,
+    alg::AuxiliaryParticleFilter,
+    iter::Integer,
+    state,
+    observation;
+    kwargs...,
+)
+    proposed_state = predict(rng, model, alg, iter, state, observation; kwargs...)
+    filtered_state, ll = update(model, alg, iter, proposed_state, observation; kwargs...)
+
+    return filtered_state, ll
+end
+
+function reset_weights!(
+    state::ParticleState{T,WT}, idxs, filter::AuxiliaryParticleFilter
+) where {T,WT<:Real}
+    # From Choping: An Introduction to sequential monte carlo, section 10.3.3
+    state.log_weights = state.log_weights[idxs] - filter.aux[idxs]
+    return state
+end
+
+function logmarginal(states::ParticleContainer, ::AuxiliaryParticleFilter)
+    return logsumexp(states.filtered.log_weights) - logsumexp(states.proposed.log_weights)
+end
diff --git a/src/algorithms/bootstrap.jl b/src/algorithms/bootstrap.jl
index ac8ed1e..a508761 100644
--- a/src/algorithms/bootstrap.jl
+++ b/src/algorithms/bootstrap.jl
@@ -1,31 +1,32 @@
 export BootstrapFilter, BF
 
-struct BootstrapFilter{RS<:AbstractResampler} <: AbstractFilter
-    N::Integer
+struct BootstrapFilter{N,RS<:AbstractResampler} <: AbstractParticleFilter{N}
     resampler::RS
 end
 
-"""Shorthand for `BootstrapFilter`"""
-const BF = BootstrapFilter
-
 function BootstrapFilter(
     N::Integer; threshold::Real=1.0, resampler::AbstractResampler=Systematic()
 )
     conditional_resampler = ESSResampler(threshold, resampler)
-    return BootstrapFilter(N, conditional_resampler)
+    return BootstrapFilter{N, typeof(conditional_resampler)}(conditional_resampler)
 end
 
+"""Shorthand for `BootstrapFilter`"""
+const BF = BootstrapFilter
+
 function initialise(
     rng::AbstractRNG,
     model::StateSpaceModel{T},
-    filter::BootstrapFilter;
+    filter::BootstrapFilter{N};
     ref_state::Union{Nothing,AbstractVector}=nothing,
     kwargs...,
-) where {T}
-    initial_states = map(x -> SSMProblems.simulate(rng, model.dyn; kwargs...), 1:(filter.N))
-    initial_weights = zeros(T, filter.N)
+) where {N,T}
+    initial_states = map(x -> SSMProblems.simulate(rng, model.dyn; kwargs...), 1:N)
+    initial_weights = zeros(T, N)
 
-    return update_ref!(ParticleContainer(initial_states, initial_weights), ref_state)
+    return update_ref!(
+        ParticleContainer(initial_states, initial_weights), ref_state, filter
+    )
 end
 
 function predict(
@@ -37,13 +38,13 @@ function predict(
     ref_state::Union{Nothing,AbstractVector{T}}=nothing,
     kwargs...,
 ) where {T}
-    states.proposed, states.ancestors = resample(rng, filter.resampler, states.filtered)
+    states.proposed, states.ancestors = resample(rng, filter.resampler, states.filtered, filter)
     states.proposed.particles = map(
         x -> SSMProblems.simulate(rng, model.dyn, step, x; kwargs...),
         collect(states.proposed),
     )
 
-    return update_ref!(states, ref_state, step)
+    return update_ref!(states, ref_state, filter, step)
 end
 
 function update(
@@ -62,5 +63,16 @@ function update(
     states.filtered.log_weights = states.proposed.log_weights + log_increments
     states.filtered.particles = states.proposed.particles
 
-    return states, logmarginal(states)
+    return states, logmarginal(states, filter)
+end
+
+function reset_weights!(
+    state::ParticleState{T,WT}, idxs, filter::BootstrapFilter
+) where {T,WT<:Real}
+    fill!(state.log_weights, -log(WT(length(state.particles))))
+    return state
+end
+
+function logmarginal(states::ParticleContainer, ::BootstrapFilter)
+    return logsumexp(states.filtered.log_weights) - logsumexp(states.proposed.log_weights)
 end
diff --git a/src/algorithms/rbpf.jl b/src/algorithms/rbpf.jl
index 4b1b700..11b47d5 100644
--- a/src/algorithms/rbpf.jl
+++ b/src/algorithms/rbpf.jl
@@ -72,7 +72,7 @@ end
 function predict(
     rng::AbstractRNG, model::HierarchicalSSM, algo::RBPF, t::Integer, states; kwargs...
 )
-    states.proposed, states.ancestors = resample(rng, algo.resampler, states.filtered)
+    states.proposed, states.ancestors = resample(rng, algo.resampler, states.filtered, algo)
 
     states.proposed.particles = map(
         x -> marginal_predict(rng, model, algo, t, x; kwargs...),
@@ -108,7 +108,7 @@ function update(
 
     states.filtered.log_weights = states.proposed.log_weights + log_increments
 
-    return states, logmarginal(states)
+    return states, logmarginal(states, algo)
 end
 
 #################################
diff --git a/src/containers.jl b/src/containers.jl
index e7e652b..e31c7f0 100644
--- a/src/containers.jl
+++ b/src/containers.jl
@@ -1,5 +1,5 @@
 using DataStructures: Stack
-using Random: rand
+import Random: rand
 
 ## GAUSSIAN STATES #########################################################################
 
@@ -105,13 +105,20 @@ Base.keys(state::ParticleState) = LinearIndices(state.particles)
 Base.@propagate_inbounds Base.getindex(state::ParticleState, i) = state.particles[i]
 # Base.@propagate_inbounds Base.getindex(state::ParticleState, i::Vector{Int}) = state.particles[i]
 
-function reset_weights!(state::ParticleState{T,WT}) where {T,WT<:Real}
+function reset_weights!(state::ParticleState{T,WT}, idx, ::AbstractFilter) where {T,WT<:Real}
     fill!(state.log_weights, zero(WT))
     return state.log_weights
 end
 
+function logmarginal(states::ParticleContainer, ::AbstractFilter)
+    return logsumexp(states.filtered.log_weights) - logsumexp(states.proposed.log_weights)
+end
+
 function update_ref!(
-    pc::ParticleContainer{T}, ref_state::Union{Nothing,AbstractVector{T}}, step::Integer=0
+    pc::ParticleContainer{T},
+    ref_state::Union{Nothing,AbstractVector{T}},
+    ::AbstractFilter,
+    step::Integer=0,
 ) where {T}
     # this comes from Nicolas Chopin's package particles
     if !isnothing(ref_state)
@@ -122,10 +129,6 @@ function update_ref!(
     return pc
 end
 
-function logmarginal(states::ParticleContainer)
-    return logsumexp(states.filtered.log_weights) - logsumexp(states.proposed.log_weights)
-end
-
 ## SPARSE PARTICLE STORAGE #################################################################
 
 Base.append!(s::Stack, a::AbstractVector) = map(x -> push!(s, x), a)
@@ -180,7 +183,7 @@ function prune!(tree::ParticleTree, offspring::Vector{Int64})
 end
 
 function insert!(
-    tree::ParticleTree{T}, states::Vector{T}, ancestors::AbstractVector{Int64}
+    tree::ParticleTree{T}, states::Vector{T}, ancestors::AbstractVector{<:Integer}
 ) where {T}
     # parents of new generation
     parents = getindex(tree.leaves, ancestors)
@@ -213,7 +216,7 @@ function expand!(tree::ParticleTree)
     return tree
 end
 
-function get_offspring(a::AbstractVector{Int64})
+function get_offspring(a::AbstractVector{<:Integer})
     offspring = zero(a)
     for i in a
         offspring[i] += 1
diff --git a/src/resamplers.jl b/src/resamplers.jl
index d0e74e5..60a7b36 100644
--- a/src/resamplers.jl
+++ b/src/resamplers.jl
@@ -8,13 +8,15 @@ export Multinomial, Systematic, Metropolis, Rejection
 abstract type AbstractResampler end
 
 function resample(
-    rng::AbstractRNG, resampler::AbstractResampler, states::ParticleState{PT,WT}
+    rng::AbstractRNG,
+    resampler::AbstractResampler,
+    states::ParticleState{PT,WT},
+    filter::AbstractFilter;
+    weights::AbstractVector{WT}=StatsBase.weights(states)
 ) where {PT,WT}
-    weights = StatsBase.weights(states)
     idxs = sample_ancestors(rng, resampler, weights)
-
-    new_state = ParticleState(deepcopy(states.particles[idxs]), zeros(WT, length(states)))
-
+    new_state = ParticleState(deepcopy(states.particles[idxs]), zeros(WT, length(states))) 
+    reset_weights!(new_state, idxs, filter)
     return new_state, idxs
 end
 
@@ -23,8 +25,9 @@ function resample(
     rng::AbstractRNG,
     resampler::AbstractResampler,
     states::RaoBlackwellisedParticleState{T,M,ZT},
+    ::AbstractFilter;
+    weights=StatsBase.weights(states)
 ) where {T,M,ZT}
-    weights = StatsBase.weights(states)
     idxs = sample_ancestors(rng, resampler, weights)
 
     new_state = RaoBlackwellisedParticleState(
@@ -49,7 +52,7 @@ struct ESSResampler <: AbstractConditionalResampler
 end
 
 function resample(
-    rng::AbstractRNG, cond_resampler::ESSResampler, state::ParticleState{PT,WT}
+    rng::AbstractRNG, cond_resampler::ESSResampler, state::ParticleState{PT,WT}, filter::AbstractFilter
 ) where {PT,WT}
     n = length(state)
     # TODO: computing weights twice. Should create a wrapper to avoid this
@@ -58,7 +61,7 @@ function resample(
     @debug "ESS: $ess"
 
     if cond_resampler.threshold * n ≥ ess
-        return resample(rng, cond_resampler.resampler, state)
+        return resample(rng, cond_resampler.resampler, state, filter; weights=weights)
     else
         return deepcopy(state), collect(1:n)
     end
diff --git a/test/runtests.jl b/test/runtests.jl
index 86c0f96..4d85a0c 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -109,7 +109,9 @@ end
     _, _, data = sample(rng, model, 20)
 
     bf = BF(2^12; threshold=0.8)
+    apf = APF(2^10, threshold=1.)
     bf_state, llbf = GeneralisedFilters.filter(rng, model, bf, data)
+    _, llapf= GeneralisedFilters.filter(rng, model, apf, data)
     kf_state, llkf = GeneralisedFilters.filter(rng, model, KF(), data)
 
     xs = bf_state.filtered.particles
@@ -120,6 +122,7 @@ end
 
     # since this is log valued, we can up the tolerance
     @test llkf ≈ llbf atol = 0.1
+    @test llkf ≈ llapf atol = 2
 end
 
 @testitem "Forward algorithm test" begin