This article presents discussion and recommendations on approaches to retrospectively evaluating development interventions in the long term through a systems lens. It is based on experiences from the implementation of an 18-month study to investigate the impact of development interventions on economic and social change over a 40-year period in the Koshi Hills region of Nepal. A multi-disciplinary team used a mixed-methods approach to data collection and analysis. A theory-based analytical approach was utilized to produce narratives of plausible cause and effect and identify key drivers of change within the context of cumulative and interconnected impacts of multiple programs and factors. This article responds to increasing interest in development evaluation to look beyond intervention-specific impact to broader determinants of change to assist with intervention planning. It is the authors' hope that this will stimulate debate and progress in the use of high quality research to inform future development work.