The present investigation has been undertaken in order to better understand the development of free, plane liquid jets. Both the development of the basic laminar flow as well as its stability have been investigated theoretically and experimentally.The velocity field and free surface location of a liquid jet emanating from a plane channel was calculated numerically and the velocity and surface relaxation lengths were determined. Calculated velocity profile distributions were in good agreement with Pitot tube measurements.Temporal linear stability calculations were performed using the calculated velocity distributions. The calculations showed five unstable modes, three sinuous and two dilatational. Four of these modes have been reported earlier and one of the sinuous modes is considered to be ‘new’. The linear stability calculations include surface tension as well as viscosity in the liquid and gas.Hot wire anemometry measurements of controlled forced disturbances showed that waves in the experimental jet also were sinuous and that the amplitude distribution was in fair agreement with theoretical results.Shadowgraph visualisations showed the evolution of the waves on the surface of the jet and it was found that the waves break up downstream the nozzle. This break-up was visualized by particle visualisations, which showed that it creates strong streamwise streaks in the jet.