The terms “polymer blends” and “polymer alloys” are defi ned in Chapters 1 and 2 of this Handbook. In the trade literature, they have been used interchangeably. In the context of current discussion, we will treat all of them simply as polymer blends, except specifying, where possible, the origin of the technological compatibility between the components in each type of blend. Table 15.1 lists the commercially available polymer blends according to their primary structural categories