Objective: The purpose of this study was to critically review and synthesize existing studies on robot motion legibility and suggest future directions for human factors research on human-robot interaction (HRI). Three questions were considered: ‘Why is the robot’s legibility important to humans?’, ‘What robot expressions improve robot’s legibility?’, and ‘How does the legibility of robots affect human performance and well-being?’. Background: As robots are quickly becoming an intrinsic part of our daily lives, the demand for technology necessary for the human-robot coexistence beings is increasing. The robot’s legibility is being increasingly recognized as an important design factor for creating human friendly robots. Method: This study critically reviewed and synthesized 30 research articles on the legibility of robot motions. Major findings and limitations of past research studies were examined and suggestions on future human factors research directions were provided. Results: From the psychology point of view, the robot’s legibility has fundamental significance concerning HRI as it is related to the basic human tendency of predicting the goal of a system from its observed behaviors. Also, designing legible robot motions can be understood as creating robot behaviors compatible with human mental models. Many studies have attempted to create highly legible behaviors through approaches, such as imitating human behaviors, considering human abilities, meeting human preferences, and using additional motion cues. Design factors such as actor characteristics, motion trajectory and velocity, were found to affect the legibility of robot. Legible robot motions were found to not only improve the task performance of the whole human-robot collaboration system but also enhance the perceived safety, comfort, trust and positive affect of robots. Conclusion: Although the robot motion legibility is an important human factors research topic concerning HRI, there are currently many research voids that require extensive future research. Application: The current literature review provides an understanding of the recent research trends on the legibility of robot motions and would help human factors researchers define future HRI research problems.