Published online by Cambridge University Press: 22 October 2009
Significant progress has been made over the last few decades in probing the gaseous component of planet-forming disks. I discuss how an understanding of the evolution of the gas in disks can help us to understand the processes of giant and terrestrial planet formation. I also discuss the observational tools that are currently available to study the gaseous component. These include in situ probes of the gas, as well as more indirect probes, such as stellar accretion rates. These tools can be used to probe the evolutionary status of various classes of young stars and thereby provide additional insights into the physical processes that govern planet formation.