This chapter provides an overview of assessment methods that are used to examine sleep. We shall focus on techniques that are commonly used to diagnose sleep disorders, evaluate treatment response and summarize investigative methods adopted by sleep scientists. The following techniques will be covered in this chapter including: polysomnography, actigraphy, brain imaging, hormone assessment, and subjective measures including self-report questionnaires and sleep diaries. These measures enable sleep scientists to further understand the role of sleep on human behavior.