Psychological assessment and psychodiagnostic investigation are central to psychologists’ work, particularly in mental health inpatient settings. This has been the case for decades and is likely to continue to be true for the foreseeable future. During the early and mid-twentieth century, international pioneers in psychology and various medical specialties initiated development of numerous measures of cognitive and psychological functioning. Numerous assessment tools were developed in order to evaluate, quantify, and better understand the workings and complexities of the human mind. These tests were initially used for a variety of purposes in a range of settings, including measurement and screening in educational, military, and immigration settings. Over time, versions of many of these measures have become important tools, which are commonly utilized in inpatient and other mental health settings.