Based on applications, ultrasonic tomography can be classified into two categories, reflective tomography and transmissive tomography. Reflective tomography offers the most potential for characterizing the shapes and sizes of cracks, voids, inclusions, corrosion, erosion, detachment of bonds, or imperfections in weldments. The underlying principle of reflective tomography is to use the Time-of-Flight information from sound pulses to determine the location of discontinuities in an otherwise homogeneous material. Transmissive tomography, on the other hand, is most effectively used for determining more general variations in density. In this case, both Time-of-Flight information and variations in sound level attenuation are used to characterize the material properties of concern. Although the end results of displaying the reconstructed images are essentially the same for the two cases, the logic and algorithms are considerably different. The systems and examples of applications of both methods are discussed.