There is an encouraging move towards making evidence-based policy decisions when deciding which development interventions to expand, to replicate with other groups, or to undertake in other contexts. In research terms, these are questions of external validity. Most applied research efforts over the last decade, however, have been almost exclusively concerned with internal validity (or enhancing the quality of causal inference pertaining to claims regarding a given project’s/policy’s impact); this is important, but even if one has supreme confidence in the veracity of one’s identification strategy for a given intervention, just how generalizable are its findings to other times, places, circumstances and scales of operation? This paper endeavors to provide practitioners with an accessible guide to external validity concerns, especially as it pertains to "complex" interventions.