Andrew's major contribution has been to tackle fundamental questions about the relationship between people and the global loss of nature - is conservation worthwhile, why is nature being lost, how much would conservation cost, and how can we achieve it efficiently? A series of innovative and well-structures studies that directly address these questions have had important implications for global conservation and for the valuation of nature. His work is characterized by detailed quantitative studies, by clever use of the data that is available and by well designed studies that effectively balance rigour with pragmatism.