Activity-based sleep-wake monitoring or actigraphy has gained a central role as a sleep assessment tool in sleep medicine. It is used for sleep assessment in clinical sleep research, and as a diagnostic tool in sleep medicine. This update indicates that according to most studies, actigraphy has reasonable validity and reliability in normal individuals with relatively good sleep patterns. The validity of actigraphy in special populations or with individuals with poor sleep or with other sleep-related disorders is more questionable. The most problematic validity issue is the low specificity of actigraphy in detecting wakefulness within sleep periods reported with certain devices or samples. Overall, the recent literature adds to previous reports in demonstrating that actigraphy is sensitive in detecting unique sleep patterns associated with specific sleep disorders as well as with other medical or neurobehavioral disorders. Furthermore, actigraphy is sensitive in detecting sleep changes associated with drug treatments and non-pharmacologic interventions. Recent developments include the development of devices specially tailored to detect periodic limb movement in sleep and the introduction of new devices and algorithms. Because of the limitations of actigraphy, it is recommended to use complementary assessment methods (objective and subjective) whenever possible.