Zeolites are a large and complex group of hydrated aluminosilicate minerals that are among the most abundant authigenic silicates in sedimentary deposits. They are so complex that only a superficial summary can be given in this short introduction, and readers are urged to consult the vastly more detailed references provided at the end of the chapter. Both water molecules and cations can be substituted or replaced in most zeolites without disrupting the crystal structure, accounting for their great compositional diversity as well as their widespread use in ion-exchange applications. Their classification and nomenclature has undergone, and continues to undergo, extensive review and revision (e.g., Coombs et al. 1998; McCusker et al., 2001). Although more than 30 different zeolites have been noted from sediments and sedimentary rock, only five are common (analcime, clinoptilolite, heulandite, laumontite and phillipsite) and three others (chabazite, erionite and mordenite) are relatively common (Gottardi and Galli, 1985; Hay and Sheppard, 2001).