Objective: With growing concerns to establish the value for returns on public health investment, there is a need to identify cost-effective physical activity interventions. This study measured change in moderate physical activity (MPA) in seven community-based intervention types, costs and cost-effectiveness of the interventions, and possible explanations for cost variation. Design: Between 2004 and 2006, pre- and post-intervention assessments identified demographics and MPA levels, and quarterly interviews with intervention managers identified key implementation costs. Setting: Nine sites across the UK. Method: MPA was collected using self-report measures. MPA categories (sedentary, lightly, moderately, highly active) were assigned at pre- and post-intervention. Differences between pre- and post-intervention scores identified MPA change (median metabolic equivalent (MET)-minutes/week) in completers. Cost, attendance and activity data were combined to estimate the average monthly implementation cost, cost per participant attending interventions, and the cost per completer improving MPA category. An economic model was built to estimate the cost per Quality Adjusted Life Year (QALY) gained and potential savings to the National Health Service (NHS). Results: Demographics (n = 984) show that those who engaged with the interventions were predominantly white, British older females. In completers (N = 1,051), 37.9 per cent improved at least one MPA category. The cost per completer improving MPA category ranged from 260 British Pounds to 2,786 British Pounds (N = 1000). The cost per QALY gained from intervention types ranged from 47 British Pounds to 509 British Pounds, which was below the 20,000 British Pounds threshold implicit in National Institute for Health and Clinical Excellence (NICE) decisions. Future cost savings to the NHS per intervention participant ranged from 769 British Pounds to 4,891 British Pounds. In the case of each of the interventions, this saving per participant exceeds the implementation cost per participant, which ranged from 55 British Pounds to 3,420 British Pounds (N = 6940). Conclusions: MPA interventions are cost-effective. (Contains 3 tables.)