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Disks that form around young stars are predominantly gaseous to begin with, and gas influences the physics and chemistry of planet formation from the first stages of grain growth to the final stage of disk dispersal. Our understanding of planet formation and disk evolution is currently hampered by our ignorance of the gas contained in disks at any of these stages. We therefore also lack an understanding of how the gas/dust ratio in the disk evolves to allow planet formation. Measuring the amount of gas present in protoplanetary disks has proven to be a formidable and challenging problem due to both a lack of suitable tracers and a highly coupled physics and chemistry. In this talk, I will describe how we can best attempt to estimate disk masses by modeling the physical and chemical structure of disks, and present recent results using models that include grain surface chemistry.