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Organic aerosols comprise a rich mixture of compounds, many generated via nonselective radical oxidation. This produces a plethora of products, most unidentified, and mechanistic understanding has improved with instrumentation. Recent advances include recognition that some peroxy radicals undergo internal H-atom transfer reactions to produce highly oxygenated molecules and recognition that gas-phase association reactions can form higher molecular weight products capable of nucleation under atmospheric conditions. Particles also range from molecular clusters near 1 nm diameter containing a few molecules to coarse particles above 1 μm containing 1 billion or more molecules. A mixture of organics often drives growth of particles. We can describe this via detailed thermodynamics, and we can also describe the physics driving mixing between separate populations containing semi-volatile organics. Finally, fully size-resolved particle microphysics enables detailed comparisons between theory and observations in chamber experiments.