This monograph describes the theory behind Raptor codes, and elucidates elements of the processes behind the design of two of the most prominent members of this class of codes: R10 and RaptorQ (RQ). R10 has already been adopted by a number of standards' bodies, and RQ is in the process of entering various standards at the time of writing of this monograph. The monograph starts with the description of some of the transmission problems, which inspired the invention of Fountain codes. Thereafter, Luby transform codes (LT codes) and Raptor codes are introduced and insights are provided into their design. These codes are currently the most efficient realizations of Fountain codes. Different algorithms are introduced for encoding and decoding various versions of these codes, including their systematic versions. Moreover, a hybrid decoding algorithm called “inactivation decoding” is introduced, which is an integral part of all modern implementations of Raptor codes. The R10 and RQ codes have been continued and will continue to be adopted into a number of standards and thus there are publicly available specifications that describe exactly how to implement these codes. However, the standards' specifications provide no insight into the rationale for the design choices made. One of the primary purposes of this document is to provide this design rationale. We provide results of extensive simulations of R10 and RQ codes to show the behavior of these codes in many different scenarios.