Publisher Summary This chapter deals with turbo codes, one of the most powerful types of forward-error-correcting channel codes. It discusses the underlying concepts and presents a description and comparison of the turbo codes used by the Universal Mobile Telecommunications System (UMTS) and cdma2000 third-generation cellular systems. Forward-error-correcting (FEC) channel codes are commonly used to improve the energy efficiency of wireless communication systems. By the end of the 1990s, the virtues of turbo codes were well known, and they began to be adopted in various systems. One of the most interesting characteristics of a turbo code is that it is not just a single code. This chapter illustrates the performance of the turbo codes used by the two third-generation cellular standards. Although turbo codes have the potential to offer unprecedented energy efficiencies, they have some peculiarities, which result in error flooring. A way to reduce the error floor is to arrange the two constituent encoders in a serial concatenation, rather than in a parallel concatenation. One solution to the synchronization problems is to incorporate the synchronization process into the iterative feedback loop of the turbo decoder itself.