Sequential Monte Carlo (SMC) methods comprise one of the most successful approaches to approximate Bayesian filtering. However, SMC without good proposal distributions struggle in high dimensions. We propose nested sequential Monte Carlo (NSMC), a methodology that generalises the SMC framework by requiring only approximate, properly weighted, samples from the SMC proposal distribution, while still resulting in a correct SMC algorithm. This way we can exactly approximate the locally optimal proposal, and extend the class of models for which we can perform efficient inference using SMC. We show improved accuracy over other state-of-the-art methods on several spatio-temporal state space models.Inference in complex and high-dimensional statistical models is a very challenging problem that is ubiquitous in applications such as climate informatics [Monteleoni et al., 2013], bioinformatics [Cohen, 2004] and machine learning [Wainwright and Jordan, 2008], to mention a few.We are interested in sequential Bayesian inference in settings where we have a sequence of posterior distributions that we need to compute. To be specific, we are focusing on settings where the model (or state variable) is high-dimensional, but where there are local dependencies. One example of the type of models we consider are the so-called spatiotemporal models [Wikle, 2015, Cressie and Wikle, 2011, Rue and Held, 2005.Sequential Monte Carlo (SMC) methods comprise one of the most successful methodologies for sequential Bayesian inference. However, SMC struggles in high dimensions and these methods are rarely used for dimensions, say, higher than ten . The purpose of the NSMC methodology is to push this limit well beyond the single digits.The basic strategy is to mimic the behavior of a so-called fully adapted (or locally optimal) SMC algorithm. Full adaptation can drastically improve the efficiency of SMC in high dimensions [Snyder et al., 2015]. Unfortunately, it can rarely be implemented in practice since the fully adapted proposal distributions are typically intractable. NSMC addresses this difficulty by requiring only approximate, properly weighted, samples from the proposal distribution. This enables us to use a second layer of SMC to simulate approximately from the proposal. The proper weighting condition ensures the validity of NSMC, thus providing a generalisation of the family of SMC methods. This paper extends preliminary work [Naesseth et al., 2015a] with the ability to handle more expressive models, more informative 2 central limit theorems and convergence proofs, as well as new experiments.