Title: Analysis of Error Feedback in Compressed Federated Non-Convex Optimization. Abstract: Communication cost between the clients and the central server could be a bottleneck in real-world Federated Learning (FL) systems. In classical distributed learning, the method of Error Feedback (EF) has been a popular technique to remedy the downsides of biased gradient compression, but literature on applying EF to FL is still very limited. In this work, we propose a compressed FL scheme equipped with error feedback, named Fed-EF, with two variants depending on the global optimizer. We provide theoretical analysis showing that Fed-EF matches the convergence rate of the full-precision FL counterparts in non-convex optimization under data heterogeneity. Moreover, we initiate the first analysis of EF under partial client participation, which is an important scenario in FL, and demonstrate that the convergence rate of Fed-EF exhibits an extra slow down factor due to the ``stale error compensation'' effect. Experiments are conducted to validate the efficacy of Fed-EF in practical FL tasks and justify our theoretical findings.