M-mode and two-dimensional echocardiography (2DE) are standard parts of the evaluation of cardiac structure and function in veterinary and human cardiology. Doppler echocardiography, although considered standard in human cardiology, is in its infancy in veterinary cardiology. It is becoming an important non-invasive component of the evaluation of cardiac patients since it adds information formerly only available from cardiac catheterization. Specifically, it can analyze blood flow direction, character, velocity, and timing. Like M-mode and 2DE, this safe, non-invasive technique is ideal for serial evaluation of patients with heart disease. Although useful in both congenital and acquired heart disease, in our hospital it has primarily been used to evaluate congenital heart defects. The purpose of this chapter is threefold. First, it will acquaint the reader with the principles of Doppler echocardiography. Second, it will give the reader an explanation of the Doppler examination. Finally, it will discuss the uses and limitations of Doppler echocardiography, with an emphasis on congenital heart disease.