The introduction of contrast imaging and harmonic imaging has revolutionized ultrasound. We will discuss the mechanism and limitations of harmonic contrast, tissue perfusion and harmonic imaging of tissue. The use of harmonics has dramatically impacted the ultrasound world. In this module, we learn the relative amplitudes of normal blood, blood with a contrast agent, harmonic signals from blood, harmonic contrast blood signals, and harmonic signals from tissue. We then discuss how harmonics are generated from tissue without the use of a contrast agent, the non-linear aspects of harmonic signal generation, the benefits of harmonic imaging, and the limitations of harmonic imaging, as well as approaches to overcome these limitations.