Open rotor noise derives from a variety of different sources that can result in tone or broadband noise. In this chapter we discuss these different sources and present analytical methods for the prediction of some of the most important for low Mach number flows. Time and frequency domain approaches to the prediction of loading noise and thickness noise are presented. Amiet's approximation for the prediction of broadband rotor noise is introduced. The chapter concludes with a discussion of the haystacking phenomenon and the prediction of rotor noise when there is significant blade-to-blade correlation.