Secondary organic aerosol (SOA) contributes substantially (15–80% by mass) to the global burden of atmospheric fine particulate matter (PM_2.5), which exerts large but uncertain effects on climate as well as adverse impacts on air quality and human health. Deciphering the molecular composition, and in turn formation and aging mechanisms, of SOA is essential to reducing uncertainty in assessment of its environmental and health impacts. However, molecular characterization of SOA, which generally consist of hundreds or more compounds of diverse classes, is significantly hindered by its chemical complexity. This seminar will present an overview of research aimed at constraining the formation, abundance, and evolution of molecular products in SOA derived from the ozonolysis and photooxidation of α-pinene and β-pinene—substantial global SOA sources—using a combination of environmental chamber and flow tube experiments, liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS), and organic synthesis.