The Feinstein International Center has been developing and adapting participatory approaches to measure the impact of livelihoods based interventions since the early nineties. Originally released in 2009, this updated version of our PIA guide takes account of the continuing need to include local people in defining and measuring impact, while also using a systematic approach to gathering evidence. The guide has been expanded to cover both humanitarian and development projects, and to take account of the growing interest in “participatory numbers”.