The decrease in mortality in critical patient units led to an increase in intensive care unit acquired weakness (ICUAW), which significantly affects the functional performance and quality of life of patients. There are several scales that measure functionality in critical patients. The aim of this systematic review is to determine the criterion validity and reliability of the scales that evaluate functionality in critically ill adult patients. We considered studies in critically ill adult subjects of both genders that determined the psychometric properties of the scales that evaluate functionality. Six minutes' walk test (6MWT), timed up and go (TUG), the Medical Research Council sum score (MRC-SS), grip strength, discharge destination and need for rehabilitation at discharge were considered as gold standards. Three scales were identified: PFIT-s, Perme MS, and DEMMI. PFIT-s has a positive correlation with 6MWT, MRC-SS and grip strength, and a negative correlation with TUG. It also predicts the need for rehabilitation at discharge and discharge to the home. DEMMI has a positive correlation with MRC-SS. the interobserver reliability was evaluated in three articles, demonstrating an almost perfect association. The intraobserver agreement was considered good in one report. With this information, it is not possible to determine which is the instrument with better measurement properties.