Assessment of functional status is a major responsibility for professionals practicing in rehabilitation facilities. Functional assessment tools have been created to meet this need. One of the most widely used tools is the Functional Independence Measure (FIM). Data from the FIM are used to examine patient outcomes for several purposes. This article explores the rationale for use of the FIM as an outcomes measure and research regarding the validity, reliability, responsiveness, and utility of this tool. Limitations of this instrument, future research needs, and implications for rehabilitation professionals also are discussed.