This chapter explores the motivations and methods for mining (collecting, aggregating, distributing, and analyzing) data about free/libre open source software (FLOSS) projects. It first explores why there is a need for this type of data. Then the chapter outlines the current state-of-the art in collecting and using quantitative data about FLOSS project, focusing especially on the three main types of FLOSS data that have been gathered to date: data from large forges, data from small project sets, and survey data. Finally, the chapter will describe some possible areas for improvement and recommendations for the future of FLOSS data collection.