In the context of this chapter, the term “dosimetric models” encompasses a number of approaches to determining the absorbed dose to humans exposed to ionizing radiation from both internal and external sources. These determinations for internal sources have been almost exclusively computational. The approaches began very simply but over time, as the computational technology evolved, so did the capabilities to produce more realistic models of the human body. Early in their development, the models were called “anthropomorphic models” (having human characteristics) but soon the term “phantom” was applied to these models. The term “phantom” will be used almost exclusively in this chapter.