How do data intermediaries know their work is making a difference? NNIP partners and other data intermediaries can answer this question by carefully defining and measuring their performance and communicating their answers to their funders and constituents. This guidebook frames defining and measuring performance for local data intermediaries. The guidebook introduces ideas about how impact can be measured for the different types of activities data intermediaries perform. It offers suggestions for an overall process of performance management and describes techniques for documenting and measuring the performance of data intermediary work.