Travel behaviour - especially car use - is of concern because it contributes to environmental problems such as climate change. Focusing on commuting, this thesis aimed to explain people's travel mode decisions and what might motivate drivers to switch modes. The literature shows that - as in the wider field of environmentallysignificant behaviour - Schwartz's norm-activation theory (NAT) and Ajzen's theory of planned behaviour (TPB) are predominant in travel psychology. Research undertaken for this thesis was based on these theories. Study 1 used logistic regression (n = 312) to test NAT and the TPB's ability to explain drivers' intentions to maintain or reduce their car use for commuting to De Montfort University (DMU). A model using variables from both theories was also tested, as was a model that added contextual variables to these psychological constructs. The model including contextual variables had the greatest predictive power (shown by Rl values). There were interactions between several predictor variables. Most notably, the influence of altruistic (pro-environmental) motives on intentions was moderated by perceived control over commuting mode choice and by contextual factors including bicycle ownership, carriage of passengers and journey time. In study 2,24 semi-structured interviews were undertaken with commuters to DMU. Using discourse analytic techniques, the prevalent stances on car use and use of other modes for travel to work were identified. Many echoed NAT and TPB constructs (e.g. moral motives, perceived control over modal choice), underlining these theories' applicability to commuting. However, other stances were also evident, most notably affective motives and habits as reasons for commuting mode decisions. People drew on various combinations of these discourses to explain their commuting behaviour. The thesis proposes a new model of commuting mode choice and suggests guidelines for interventions designed to encourage drivers to use alternative modes. However, it is stressed that reliance on attitude-behaviour research alone may ignore wider sociocultural influences on travel behaviour. Suggestions are made regarding theoretical perspectives and methods that may help in understanding these forces and a case is made for mixed-method research as the way ahead for travel psychology.