This entry contains an extension to the Isabelle library for fixedwidth machine words. In particular, the entry adds quickcheck setup for words, printing as hexadecimals, additional operations, reasoning about alignment, signed words, enumerations of words, normalisation of word numerals, and an extensive library of properties about generic fixed-width words, as well as an instantiation of many of these to the commonly used 32 and 64-bit bases. In addition to the listed authors, the entry contains contributions by Nelson Billing, Andrew Boyton, Matthew Brecknell, Cornelius Diekmann,Peter Gammie, Gianpaolo Gioiosa, David Greenaway, Lars Noschinski, Sean Seefried, and Simon Winwood.