Several approaches are available to model the formation of secondary organic aerosols (SOA). The most commonly used approaches include the fitted-yield approach (also referred to as the two-product Odum approach), the volatility basis-set (VBS) approach, and the hydrophobic/hydrophilic organic (H2O) aerosol approach. The pros and cons of these three approaches will be discussed. Historically, most models have used the fitted-yield approach. Over the past few years, the VBS approach has been increasingly used for applications in North America and Europe, whereas the H2O approach has been developed further to be compatible with any gas-phase mechanism and has been incorporated in several European air quality models. Examples will be given for regional model simulations and compared to available measurements. Differences between SOA yields in smog chamber experiments and the ambient atmosphere will be discussed with a focus on isoprene photo-oxidation. Recommendations for further model development will be proposed.