Optical parametric oscillators (OPOs) are powerful solid-state sources of broadly tunable coherent radiation. Since the recent improvement of intense pump sources and the availability of high quality nonlinear optical materials OPOs can routinely be operated in the UV, VIS and IR spectral range with conversion efficiencies exceeding 30%. Therefore OPOs offer a great potential for lidar measurements and, due to their solid-state character, in particular for airborne lidar applications. We are currently investigating OPOs for an airborne differential absorption lidar (DIAL) system for water vapor monitoring. Any detailed understanding of the role of water vapor in the global climate system requires precise measurements in the atmosphere with high temporal and spatial resolution. The DIAL technique applied in the near infrared spectral range is generally considered to be appropriate for this purpose. In order to accurately determine atmospheric gas concentrations by means of the DIAL technique tunability and spectral properties of the lidar transmitter are very important. The stringent requirements can be achieved with an OPO by employing the technique of injection seeding with radiation from a dye laser or from a diode laser. At the same time, the diode laser is an ideal tool for the spectroscopic analysis of the absorption lines.