This IDS Bulletin explores new frontiers in international development evaluation, making a useful contribution to an ongoing debate about how to assess effects and effectiveness without ignoring the complexity of the contemporary development landscape.

Bringing together two themes – impact of development interventions and implications of development within complex systems and settings – this issue gives prominence to both systems thinking and complexity science, two perspectives increasingly drawn on by evaluators. This is the second of two IDS Bulletins (the first one was Rethinking Impact Evaluation for Development) following a workshop entitled Impact, Innovation and Learning: Towards a Research and Practice Agenda for the Future, held in March 2013 at IDS.

Straightened budgets and accountability-driven demands to demonstrate the effectiveness of public expenditure have led to the emphasis on impact evaluations. Policymakers are interested in 'evidence-based policy', while also judging the effectiveness of specific interventions, though achieving clarity and measurement of policy impacts is challenging. Systems thinking and complexity science draw on diverse roots (epistemological/technological/mathematical) and are new to development evaluation (which has traditionally favoured linear frameworks). This issue offers a view of how complexity and systems thinking could inform impact evaluation (though further research is needed to engage more fully with the conceptual and methodological possibilities that this area of work holds for evaluation).

These two issues of the IDS Bulletin represent a useful step in the right direction of incorporating systems and complexity ideas into the impact evaluator's toolkit. Their methods and agendas should stimulate insightful, conceptually sophisticated as well as practice-grounded debate.