This research examined the role of behavioural avoidance on excessive worry and applied a behavioural activation treatment to excessive worriers. The first study examined the latent construct of worry across adults from a university population (N: 536) with consideration of direct and indirect relationships with emotional distress, problem solving orientation, cognitive avoidance and behavioural avoidance. Path analysis from this model highlighted a central role for behavioural avoidance to facilitate distress in worry. Clinical implications for this study suggested that targeting behavioural avoidance in excessive worriers may benefit treatment outcome. In the second study, Behavioural Activation treatment was applied to seven excessive worriers. Treatment targeted behavioural avoidance e.g. procrastination, excessive checking of family members' safety, delay in checking exam results. Treatment strategies included functional analysis to break down avoidance patterns and ongoing activation of goal orientated behavioural steps. This seven- week group-based treatment intervention appeared to have transdiagnostic potential by producing reduced scores in excessive worry as well as depression and anxiety symptoms. 43% of participants demonstrated clinically significant change on the primary outcome of excessive worry and 57% no longer met criteria for GAD diagnosis at the end of treatment. The third study described a treatment trial of Behavioural Activation for Worry compared to a waitlist control. Treatment was delivered to 49 participants, in groups of six-eight individuals. Treatment sessions were extended to eight weeks. Twice as many individuals in the treatment group reported clinical significant reductions compared to the waitlist group at the end of the intervention. Significant improvements were also seen in life functioning and GAD symptoms. Results were maintained at four week follow-up. Regression results also demonstrated that reduction in behavioural avoidance was the best predictor of excessive worry four weeks after treatment completion, highlighting the key role of behavioural avoidance in the treatment of excessive worry.