Title: Policy Optimization by Local Improvement through Search. Abstract: Imitation learning has emerged as a powerful strategy for learning initial policies that can be refined with reinforcement learning techniques. Most strategies in imitation learning, however, rely on per-step supervision either from expert demonstrations, referred to as behavioral cloning or from interactive expert policy queries such as DAgger. These strategies differ on the state distribution at which the expert actions are collected -- the former using the state distribution of the expert, the latter using the state distribution of the policy being trained. However, the learning signal in both cases arises from the expert actions. On the other end of the spectrum, approaches rooted in Policy Iteration, such as Dual Policy Iteration do not choose next step actions based on an expert, but instead use planning or search over the policy to choose an action distribution to train towards. However, this can be computationally expensive, and can also end up training the policy on a state distribution that is far from the current policy's induced distribution. In this paper, we propose an algorithm that finds a middle ground by using Monte Carlo Tree Search (MCTS) to perform local trajectory improvement over rollouts from the policy. We provide theoretical justification for both the proposed local trajectory search algorithm and for our use of MCTS as a local policy improvement operator. We also show empirically that our method (Policy Optimization by Local Improvement through Search or POLISH) is much faster than methods that plan globally, speeding up training by a factor of up to 14 in wall clock time. Furthermore, the resulting policy outperforms strong baselines in both reinforcement learning and imitation learning.