Titan exhibits ample surface and crustal processes including lakes and seas, fluvial erosive features, possibly subsurface reservoirs of liquid, and rainfall. Together these constitute strong evidence for a multicomposition hydrological system, composed mostly of methane and ethane as well as trace amounts of other alkanes. Estimates of the volume of liquid methane required in streams and rainfall to produce erosional features suggest that these could be relatively recent phenomena, perhaps periodically renewed as the overall climate cycles between dry and wet periods. The end state of the longer term chemical processing of methane in the upper atmosphere is expressed on the surface in the form of deposits of solid organics organized into dunes, and lighter hydrocarbons such as ethane (in the lakes), acetylene, and other hydrocarbons and nitriles.