The evolution of primordial circumstellar disks

circumstellar disks are an integral part of the star formation process and the sites where planets are formed. Understanding the physical processes that drive their evolution, as disks evolve from optically thick to optically thin, is crucial for our understanding of planet formation. Disks evolve through various processes including accretion onto the star, dust settling and coagulation, dynamical interactions with forming planets, and photo-evaporation. However, the relative importance and timescales of these processes are still poorly understood. In this review, I summarize current models of the different processes that control the evolution of primordial circumstellar disks around low-mass stars (mass < 2 M circle dot). I also discuss recent observational developments on circumstellar disk evolution with a focus on new Spitzer results on transition objects.