Functional assessment is a technique consisting of a set of procedures that define the relationship between events in the environment and specific target behaviors. The purpose of functional assessment is to identify maintaining contingencies for challenging behavior in an individuals environment. This chapter provides an overview of populations evaluated with functional assessment, which includes the following: Autism Spectrum Disorder, Intellectual Disability, Emotional Behavioral Disorder, Attention-Deficit/Hyperactivity Disorder, Fragile X Syndrome, Angelman Syndrome, Prader-Willi Syndrome, Smith-Magenis Syndrome, Lesch-Nyhan Syndrome, Acquired Brain Injury, Typically Developing Children, Children at Risk of Developmental Disabilities, Children with Prenatal Drug Exposure, and Children who use Wheelchairs. The subsequent part of this chapter will focus on the behaviors and problems evaluated with functional assessment, which include the following: aggression, self-injurious behavior, stereotypy, bizarre speech, skin picking, hand mouthing, feeding problems, elopement, noncompliance and disruptive behavior, multiple typographies of problem behavior, sleep problems, and happiness behaviors. An extensive review of the research will provide the reader with the range of possible uses of functional assessment.