Concepts for lighter-than-air exploration of Titan are reviewed. Logical vehicle designs with existing power sources (primary batteries, or 100 We-class radioisotope power supplies) are strongly bifurcated into small (few kg) vehicles with lifetimes of only a few hours/days, or larger (100 kg+) vehicles with lifetime of months to years. A wide range of the latter class of vehicle exists, including buoyant-gas and hot-air (Montgolfi�re) concepts, with and without altitude control or propulsion. An attractive intermediate (10s of kg, but long-lived) class of buoyant gas balloon would be enabled by small (0.5-10 We) radioisotope power supplies, not presently available in the USA. The marginal feasibility of high altitude solar-powered balloons is discussed.