This chapter introduces cavity theory in radiation dosimetry through a simple derivation and explores its application in absorbed dose measurements, along with correction methods that improve accuracy when the conditions required for the theory are not fully met. The use and limitations of Bragg–Gray and Spencer–Attix theory are described in the context of megavoltage beams. Monte Carlo simulation is introduced using a simple example, and the theoretical tests used to validate its accuracy are presented. The application of Monte Carlo methods in modeling ionization chamber detectors is discussed, highlighting its impact on improving the accuracy of reference dosimetry. With increased computing capabilities, the chapter illustrates the growing role of Monte Carlo techniques in enhancing dosimetric precision.