  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. I also discuss recent observational
developments on circumstellar disk evolution with a focus on new Spitzer
results on transition objects.
