The title ofthis book， Channel Codes： Classical and Modern，was selected to reflect the fact that this book does indeed cover both classical and modern channel codes，It includes BCH codes， Reed-Solomon codes， convolutional codes， finite-geometry codes， turbo codes， low-density parity-check (LDPC) codes， and product codes.However， the title has a second interpretation. While the majority of this book is on LDPC codes， these can rightly be considered to be both classical (having been first discovered in 1961) and modern (having been rediscovered circa 1996). This is exemplified by David Forney's statement at his August 1999 IMA talk on codes on graphs，"lt feels like the early days." As another example of the classical/modern duality， finite-geometry codes were studied in the 1960s and thus are classical codes. However， they were rediscovered by Shu Lin et a/. circa 2000 as a class of LDPC codes with very appealing features and are thus modern codes as well. The classical and modern incarnations of flnite-geometry codes are distinguished by their decoders： one-step hard-decision decoding (classical) versus iterative soft-decision decoding (modern).