Maximum medical improvement (MMI) and its multiple synonyms are important terms to understand because one cannot determine permanent impairment until a condition has resolved or reached a stable plateau with respect to improvement. Further, in many jurisdictions MMI represents the date beyond which temporary or all benefits cease or time-loss compensation (temporary disability benefits) may be terminated. Because each arena is to some extent unique, evaluating physicians should become familiar with MMI or the term used and its definition in the applicable federal, state, or provincial law or insurance policy. A table shows the terminology used by various US workers’ compensation jurisdictions, but there is no universal definition for MMI. The fourth edition of the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) says only that “an impairment should not be considered ‘permanent’ until the clinical findings, determined during a period of months, indicate that the medical condition is static and well stabilized. The AMA Guides, Fifth Edition, defines MMI as “a condition or state that is well stabilized and unlikely to change substantially in the next year, with or without medical treatment” but acknowledges the possibility of “some change.” Similarly the sixth edition states that MMI is the “point at which a condition has stabilized and is unlikely to change (improve or worsen) substantially in the next year, with or without treatment”; signs and symptoms may wax and wane, but further recovery or deterioration is not anticipated.