Clinical guidelines are a feature of any health care systems.  They have the potential to improve clinical practice, but a well-developed evidence base to guide the choice of strategies to disseminate and implement guidelines is currently lacking. Psychology has produced theories that have been successful in explaining behaviour and behaviour change across circumstances.  They help to explain how interventions to implement guidelines operate to change clinical practice (a form of behaviour), and the circumstances in which interventions may be effective. Aims: 1)  To explore the theoretical base of dissemination and implementation strategies currently used in research studies; 2)  To consider ways of using psychological theories in implementation research. Study 1: A review of theory use in a systematically selected sample of 235 studies of interventions to disseminate and implement guidelines.  Studies were classified according to level and stage of theory use.  Fifty-three studies (23%) were judged to have used theories, of which fourteen (6%) were explicitly theory-based.  In the majority of cases theory was used to guide the choice or design of interventions. Study 2: The Theory of Planned Behaviour (The TPB) was used alongside a trial of an intervention to implement preventive health care practice among Canadian family physicians (n = 73). The TPB explained 54% and 34% of the variance in intentions to offer lifestyle counselling and screening tests respectively.  Physicians possessed strong attitudes and intentions towards offering preventive procedures to patients but perceptions of control over the behaviours were lower.  Use of the TPB to guide the choice, design and evaluation of interventions to change practice was considered. Conclusion: Psychological theories represent a useful tool for the development of effective interventions to implement clinical guidelines.  More studies that make explicit use of theory are needed.  Recommendations for the conduct of theory-based research are discussed.