The Biosphere Effects on AeRosols and Photochemistry Experiment (BEARPEX) was a multi-investigator collaboration conducted as two eight week deployments, August–October 2007 and June–July 2009. The measurements were among the most comprehensive suite of gas and aerosol properties attempted at a single site. The project built on an extensive background of previous experiments in the foothills of the Sierra Nevada. Our goals were to develop an improved mechanistic understanding of chemical and ecological processes affecting atmospheric composition and to use this improved understanding to provide a clearer and more accurate evaluation of how chemical processing of biogenic and anthropogenic emissions affects the composition and radiative properties of the atmosphere on urban, regional and global scales. The experimental design paid particular attention to three crosscutting issues: a) chemistry downwind of urban areas where there is high VOC reactivity (due to emissions from forests) and low NOx, b) understanding the full oxidation sequence and subsequent fate of biogenic VOC and c) the processes leading to formation and removal of biogenic secondary organic aerosol (SOA) and the associated physical, chemical and optical properties of SOA. Results from the BEARPEX campaign will be published in this Special Issue of Atmospheric Chemistry and Physics.