The current article discusses assertiveness training, a once highly popular area of investigation that has been neglected in recent years by the field of psychotherapy. A substantial body of research indicates that assertiveness is a relevant factor associated with a variety of clinical problems, populations, and contexts, and that assertiveness training is a valuable transdiagnostic intervention. Despite its demonstrated importance, research on assertiveness and assertiveness training as a standalone treatment within clinical psychology has diminished drastically. We review the history of assertiveness training, revisit early research evidence for assertiveness training in treating various clinical problems, discuss the current status of assertiveness training, consider issues of clinical implementation, and comment on how the variables accounting for unassertiveness map onto the NIMH RDoC funding priorities.