Treatment manuals were first developed in the 1950s and quickly became an important tool, allowing for the objective and replicable definition of a given psychological approach. Manuals greatly enhanced the rigor of treatment research studies, and they also provided an effective way to communicate treatment strategies to clients and clinicians at large. There are now dozens of manuals available, representing a range of problem areas and theoretical approaches. Although manuals have many advantages when compared to unstructured treatments, they have faced criticism from some practitioners. Nevertheless, manuals will continue to play an important role in psychological treatment in the future.