|An assumption in Cognitive Psychology, which has been challenged in recent years, is that the systems responsible for action and perception work independently of one another. These systems work together during conceptual tasks and research has demonstrated that action knowledge can influence performance even when the task is ‘action-irrelevant’ (Borghi, 2004; Borghi, Flumini, Natraj & Wheaton, 2012; Creem & Proffitt, 2001; Tucker & Ellis, 1998, 2001). However, participants in such tasks are often only asked to make simple category judgements, such as natural versus man made. The research reported in this thesis has shown that, under certain conditions, participants use action knowledge to make ‘complex’ category choices in an action-irrelevant task. The experimental work has predominantly used the forced-choice triad task to assess the circumstances under which participants categorise objects based on shared actions. The triads were designed with a target object and two choice objects matching on either shared actions (rifle + water pistol), shared taxonomic relations (rifle + sword), or both (orange + banana). The context in which the objects were presented was also manipulated so that the objects were either presented on a white background (context-lean) or being used by an agent (context-rich). Participants were most likely to select the choice object that shared both a taxonomic and an action demonstrating that action has an ‘additive’ effect in categorical decisions. Presenting the objects being used by an agent in a functional scenario increased the saliency of the shared actions between the stimuli, and participants were more likely to select the action choice. The subsequent experimental work reported in the thesis sought to eliminate potential confounding variables including perceptual features, object typicality and task instructions. What the experimental work presented here has demonstrated is that action can influence decisions on more complex categories, and judgments of similarity. The research has identified three main circumstances under which knowledge of action becomes influential in the triad task designed for the purpose of this research as follows: (i) when it is presented in conjunction with taxonomic information, (ii) when it is presented with a context, and (iii) when participants are first asked to physically interact with the objects.