We begin this module by learning basic Doppler theory including deriving the Doppler equation from a discussion of the Doppler Effect. After developing the Doppler equation, we discuss how the Doppler equation is applied in ultrasound. We will develop the Doppler system block diagram so that Doppler controls can be understood in the greater context of performing a clinical exam. In this section we will discuss the Doppler controls of transmit, receiver gain, signal detection, the Doppler wall filters, video compression, the PRF, Doppler spectral processing, and the spectral display. After we have completed the block diagram we will then be ready to discuss basic spectral interpretation, angular effects, and the parameters affecting aliasing.