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In this chapter, we'll look at the fundamental laws governing wave dynamics and see how ultrasound artifacts are created. Ultrasound machines rely on several physical assumptions to assign the location and intensity of each received echo. These assumptions are that the received echoes have originated from within the main ultrasound beam, an echo returns to the transducer after a single reflection, the depth of an object is directly related to the amount of time for an ultrasound pulse to return to the transducer, the speed of sound in tissue is constant (1540 m/sec), the sound beam and its echo travel in a straight path, and the acoustic energy in an ultrasound field is uniformly attenuated (Feldman et al. 2009). Artifacts may be grouped by the most important principles leading to their formation, including attenuation, velocity or propagation, and artifacts associated with multiple echoes. Many artifacts can be grouped by their association with air or fluid, making learning them easier.