Significant efforts have been made in the last decade to develop evidence-based guidelines for the treatment of insomnia and other sleep disorders. Despite such progress, there are still no standard assessment methods to document outcome and no accepted criteria to define what should be a successful outcome in the treatment of insomnia. This paper reviews methodological and conceptual issues related to the measurement of outcomes in clinical trials of insomnia. Selected studies of behavioral and pharmacological therapies of insomnia are summarized to illustrate the types of dependent variables and assessment instruments used to document treatment efficacy. Additional outcome variables and assessment methods of potential interest are discussed, and criteria for interpreting and reporting outcomes are summarized. As most studies have relied on fairly narrow criteria to define an effective treatment, the need to broaden the scope of outcome assessment is highlighted. For instance, it is essential to document treatment efficacy beyond the simple reduction of insomnia symptoms and to incorporate additional indicators of success. Given that insomnia is associated with significant morbidity, an effective treatment should not only improve sleep parameters, but it should also produce clinically meaningful changes in daytime functioning, fatigue, mood, and quality of life. The need to evaluate outcome from multiple perspectives and to develop a core-assessment battery that would consider efficacy, clinical significance, and cost-effectiveness are discussed.