Behavioural Activation (BA) therapy is a stand-alone evidence-based treatment for depression and also is being applied to anxiety with promising outcomes. Essentially, BA involves structured therapy aimed at increasing the amount of activity in a person's daily life, so that he or she comes into contact with sources of positive reinforcement for clinically healthy behaviours. Originally, contemporary BA was developed as a behaviour therapy treatment condition in a study that compared BA to Cognitive Behavioural Therapy (CBT). Over time, many variants of BA have appeared in the published literature, which included techniques that might be viewed as being incompatible with the original intended treatment model and more similar to generic forms of CBT. The purpose of this article is to provide researchers and practitioners with a description of what we consider to be the distinctive and essential elements of BA therapy.