Applied behavior analysis (ABA), also referred to as behavioral engineering, is a psychological discipline that uses respondent and operant conditioning to change human and animal behavior. ABA is the applied form of behavior analysis; the other two are: radical behaviorism (or the philosophy of the science) and experimental analysis of behavior, which focuses on basic experimental research.

The term applied behavior analysis has replaced behavior modification because the latter approach suggested changing behavior without clarifying the relevant behavior-environment interactions. In contrast, ABA changes behavior by first assessing the functional relationship between a targeted behavior and the environment, a process known as a functional behavior assessment. Further, the approach seeks to develop socially acceptable alternatives for maladaptive behaviors, often through implementing differential reinforcement contingencies.

Although ABA is most commonly associated with autism intervention, it has been used in a range of other areas, including applied animal behavior, substance abuse, organizational behavior management, behavior management in classrooms, and acceptance and commitment therapy.

ABA is controversial and rejected by the autism rights movement due to a perception that it emphasizes normalization instead of acceptance, and a history of, in some forms of ABA and its predecessors, the use of aversives, such as electric shocks.

ABA is an applied science devoted to developing procedures that will produce meaningful changes in behavior. It is to be distinguished from the experimental analysis of behavior, which focuses on basic research, but it uses principles developed by such research, in particular operant conditioning and classical conditioning. Both branches of behavior analysis adopt the viewpoint of radical behaviorism, treating thoughts, emotions, and other covert activity as behavior that is subject to the same responses as overt behavior. This represents a shift away from methodological behaviorism, which restricts behavior-change procedures to behaviors that are overt, and was the conceptual underpinning of behavior modification.

Behavior analysts emphasize that the science of behavior must be a natural science as opposed to a social science. As such, behavior analysts focus on the observable relationship of behavior with the environment, including antecedents and consequences, without resort to "hypothetical constructs".

The field of behaviorism originated in 1913 by John B. Watson with his seminal work "Psychology as the Behaviorist Views it." In the article, Watson argued against the field of psychology's focus on consciousness and proposed that the field instead focus on observable behaviors, a concept referred to as methodological behaviorism.

In the 1930s, B. F. Skinner established the concept of radical behaviorism which extended Watson's theory to encompass private events that are unobservable to others, such as thoughts and emotions.