Title: MALIBO: Meta-Learning for Likelihood-free Bayesian Optimization. Abstract: Bayesian Optimization (BO) is a popular method to optimize expensive black-box functions. Typically, BO only uses observations from the current task. Recently proposed methods try to warm-start BO by exploiting knowledge from related tasks, yet suffer from scalability issues and sensitivity to heterogeneous scale across multiple datasets. We propose a novel approach to solve these problems by combining a meta-learning technique and a likelihood-free acquisition function. The meta-learning model simultaneously learns the underlying (task-agnostic) data distribution and a latent feature representation for individual tasks. The likelihood-free BO technique has less stringent assumptions about the problems and works with any classification algorithm, making it computation efficient and robust to different scales across tasks. Finally, gradient boosting is used as a residual model on top to adapt to distribution drifts between new and prior tasks, which might otherwise weaken the usefulness of the meta-learned features. Experiments show that the meta-model learns an effective prior for warm-starting optimization algorithms, while being cheap to evaluate and invariant to changes of scale across different datasets.