Apart from the Algonquian, Muskogean, and Gulf languages, the three language families that will be discussed in this chapter account for most of the aboriginal languages spoken in North America east of the Rocky Mountains. They are treated under a single heading because of the likelihood that they are remotely related, forming a linguistic unit which might be referred to as ‘Macro-Siouan’. The first part of the chapter will be taken up with discussions of the work that has been done so far on the languages of these families. The Caddoan languages will be discussed first, then the Iroquoian, and finally the Siouan. The author is more directly familiar with the Caddoan and Iroquoian languages and with past and present linguistic work conducted in those areas. For that reason it is possible that his treatment of the Siouan languages is less complete, and he would be happy to be informed of relevant information that he has not included. The latter part of the chapter is concerned with remote relationships, particularly the Macro-Siouan hypothesis, and presents for the first time certain evidence tending to link Caddoan with both Siouan and Iroquoian.