Dairy cattle breeding programs are complex, and selection indices have become essential tools for making sound genetic decisions. There has been considerable growth over the last decade in both the number of traits included in selection indices and the selection tools available to dairy farmers through genetic evaluation centers, cattle breed associations, genetics companies, and other service providers. Selection indices combine information about many traits into a single number that can be used for ranking animals and making mating decisions. Key topics discussed in this chapter include the definition of a selection index; the way in which a selection index is constructed; current example of selection indices used in the dairy industry; and anticipated future developments in selection objectives. Other topics include alternative approaches for deriving weights, changes in selection indices over time, frequency of revision, and the perspectives of farmers and other groups as they relate to selection goals.