Interest in understanding the association between attention-deficit/hyperactivity disorder (ADHD) and sleep is nothing new. In fact, restless sleep was a symptom criterion for attention deficit disorder in the Third Edition of the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-III), and the negative impact of inadequate or insufficient sleep on attentional functioning has likewise been noted for some time. Despite the large body of research examining sleep and ADHD, our understanding of this association is far from being put to bed, so to speak. Recently, research has shifted away from simply documenting differences in sleep functioning between individuals with and without ADHD, which while important, does little to inform our understanding of the causes, correlates, and consequences of these sleep difficulties. An emerging body of research using longitudinal methodologies is revealing that the associations between sleep and ADHD are complex, and that the clinical implications of sleep difficulties in individuals with ADHD likely varies across development. [This paper was published in "The ADHD Report" v25 p7-13, 16 2017.]