This chapter provides an overview of the present understanding of jet noise from both an experimental and analytical viewpoint. First, a general review of experimental observations is provided. Only single axisymmetric jets are considered. Then a historical review of theoretical contributions to jet noise understanding and prediction is provided. The emphasis is on both the assumptions and shortcomings of the approaches, in addition to their successes. The present understanding of jet noise generation mechanisms and noise predictions is then presented. It is shown that there remain two competing explanations of many observed phenomena. The ability of the different approaches to predict jet noise is assessed. Both subsonic and supersonic jets are considered. Finally, recent prediction methods and experimental observations are described.