Childbirth is a life event that negatively influences mothers’ physical activity (PA) levels and is identified as a teachable moment for health behaviour change and therefore interventions to increase postnatal PA are required. This thesis broadly follows the first two steps in the Medical Research Council (MRC) intervention development guidance, combined with methods from the Behaviour Change Wheel (BCW). The first study systematically reviewed the existing literature on the effectiveness of postnatal PA interventions. Eleven studies were included in the narrative review and eight in the meta-analysis. There was a small but significant increase in PA behaviour in the intervention group compared to the control group, but heterogeneity was high. A need for interventions with larger sample sizes, longer follow-up periods and objective PA measurements was identified. Study two utilised a multi-methods design to explore the factors that influence postnatal PA according to the COM-B model of behaviour. Semi-structured interviews qualitatively explored participants’ views on what factors influenced PA, and a questionnaire determined their relative importance. Qualitative findings indicated that all COM-B components influenced behaviour, and quantitative findings indicated that the most important factors that influenced behaviour were time, feeling tired, lack of available childcare, lack of advice from a healthcare professional, lack of motivation and development of a habit. The results are presented in a behavioural analysis for postnatal PA. The next section of this thesis described the remaining steps of the BCW to identify intervention options, content and implementation options resulting in ‘Buddy Up’, an intervention that matches two new mothers as PA buddies to provide mutual support to increase PA. A buddy is an existing friend or another eligible participant. The intervention includes three PA counselling sessions based on Motivational Interviewing principles supplemented by a booklet. The final study explored the feasibility of delivering ‘Buddy Up’ utilising a single group pre-post study design. The study explored the feasibility of recruitment, data collection, intervention acceptability and preliminary efficacy data. 44 participants (existing friends (n=22); new match (n=22)) were recruited, and 21 participants 5 remained unmatched. Key recruitment challenges were engaging Children’s Centres (CCs) with recruitment and matching participants. Participants engaged in PA with their buddy on 1.06 days (SD=1.76) in the past week and provided support by sending encouraging messages (85.7%), sharing PA ideas/information (71.4%) and doing PA together (60%). Findings from the post-intervention interviews suggest good acceptability of the intervention sessions, minimal usage of the booklet and varied views on the acceptability of the buddy element among participants. Preliminary effectiveness data is promising for objective (Baseline=697.68 counts per minute (cpm); Follow-up=765.05 cpm) and self-report PA (Baseline=1533.56 MET-min/week; Follow-up=1917.50 MET-min/week) and has a significant effect on self-efficacy to overcome some barriers to PA (when feeling depressed, when there is no one to be physically active with, during bad weather and when they have no money). Collectively, this thesis describes the intervention development process and presents the first buddy intervention for postnatal physical activity. The feasibility study findings show promise that this is a fruitful research avenue, but the intervention’s operational feasibility requires further refinement prior to recommending a large-scale efficacy trial.