Context Comprehensive evaluation of biodiversity conservation programs is essential for informing their development as well as the design of future programs. Such evaluations should not be limited to whether targets have been met, but should also assess the cost and efficiency of meeting targets, and any factors contributing to success or failure. Aims We aimed to evaluate the effectiveness and efficiency of individual-species conservation programs, and the biological and operational factors affecting these. We used the species action plans (SAPs) within the UK Biodiversity Action Plan as our case study. Methods We used cost-effectiveness analysis, cost-utility analysis and threat-reduction assessment to evaluate the effectiveness and efficiency of individual SAPs. Then we used statistical models to investigate the relative importance of biological and operational factors affecting cost, effectiveness and efficiency. Key results Conservation plan success was affected by both biological and operational factors. Invertebrate plans tended to be less effective, whereas vertebrate plans were less efficient. Plans for widely distributed species with longer generation times tended to be less efficient. Of the three different evaluation approaches, cost-effectiveness analysis offered the best combination of ease of data collection and accuracy of data content. Conclusions The most successful SAPs concerned species with short generation times and narrow distributions. Operationally, the most successful SAPs were concise and focussed and showed clear lines of responsibility for implementation. Implications Techniques such as cost-effectiveness analysis, cost-utility analysis and threat reduction assessment should be used to inform decisions on maximising the rate of return on conservation investments, although broader ecological implications and socio-cultural benefits should also be considered. The success of conservation plans is influenced by both biological and operational factors. Because biological factors cannot be controlled or altered, where species exhibit characteristics that are likely to make their conservation less effective or efficient, it is critical that operational factors are optimised. High-quality data are necessary to underpin prioritisation decisions, and monitoring to deliver reliable data on both the benefits and costs of conservation should form a core component of conservation programs.