Publisher Summary This chapter presents the basic characteristics of axial-flow compressors and ducted fans. It also covers the analysis of axial-flow compressors stage (mean-line analysis, velocity diagrams, thermodynamics, and stage loss relationships and efficiency) and axial-flow compressors rotors. Many axial compressors are multi-stage devices and repeating stages are initially assumed in which the velocity triangles for all stages are similar, the mean radius is constant, and the axial velocity through the machine is constant. The chapter covers the main preliminary design considerations such as stage loading, flow coefficient, reaction, inter-stage swirl, and blade aspect ratio. This includes a new presentation of the ways through which the measurements of cascade loss and turning can be translated into the performance of a compressor stage. Both incompressible and compressible cases are covered in the chapter and the huge importance of off-design performance is presented in detail including the ways a designer can influence compressor operating range during the very early design stages. For preliminary design and analysis purposes, a multi-stage compressor is thought of as a series of single-stage compressors, each performing as it would in isolation. However, to understand the performance of a real machine, the behavior of the overall system must be considered in more detail. This is particularly important to understand the sources of loss in a compressor and the off-design operation.