Review Objective: The objective of this review is to synthesise the best available evidence on the effectiveness of interventions using motivational interviewing (MI) to modify dietary and physical activity behaviours. Inclusion criteria: Types of Participants: This review will consider studies that include adults, aged 18 years and older. The review will report data separately for: 1. women only interventions (or where data has been reported separately for women) 2. men only interventions (or where data has been reported separately for men) 3. interventions which do not separate results by gender. Types of interventions: This review will consider quantitative studies that investigate the effectiveness of MI intervention and interventions employing components of MI on dietary and physical activity behaviour change. Studies must separate subjects into at least 1 group receiving the MI based intervention and one group receiving an attention control.The intervention may either use MI as the sole intervention strategy or as an intervention component. The review will be divided into studies examining: 1. Dietary modification 2. Physical activity modification. Types of outcomes: This review will consider studies that include the following outcome measures: Primary outcomes: - Dietary: E.g. Energy intake, intake of core food groups, macronutrient composition of the diet, diet quality or diet index. - Activity: E.g. Physical activity or sedentary behaviours: energy expenditure, steps taken per day, time spent participating in physical activity, sitting time.