This study examines factors affecting the implementation by primary care practitioners (nursing, education, allied health, and medical) of a brief parenting and family support intervention (the Primary Care Triple P-Positive Parenting Program) following professional training. It assesses the impact of prior experience, self-efficacy, program supports, program barriers, satisfaction with training, and workplace characteristics on reported extent of program use. The majority of practitioners (97%) reported using Triple P following training. Implementation was assessed as the proportion of cases seen who received the full program. Program supports (quality of format and materials) and barriers (management difficulties and lack of fit) impacted on practitioner self-efficacy, and higher self-efficacy was positively associated with implementation. Prior professional experience, satisfaction with training, and workplace factors were not significant predictors. These results highlight the importance of promoting practitioners' sense of competence or mastery of a program for facilitating the implementation of evidence-based programs in primary care settings.