The GLM procedure and many other SAS/STAT procedures label the tests and Sums of Squares (SS) associated with the various effects in the model as Type I, Type II, Type III, and Type IV. These four types of hypotheses may not always be sufficient for a statistician to perform all desired hypothesis tests, but they should suffice for the vast majority of analyses. This chapter explains the hypotheses tested by each of the four types of SS. For additional discussion, see Freund, Littell, and Spector (1991) or Milliken and Johnson (1984).