A variety of techniques was used to assess the validity of the Descriptive Tests of Mathematics Skills (DTMS) for making placement decisions. The DTMS is a group of four tests (Arthmetic Skills, Elementary Algebra Skills, Intermediate Algebra Skills, and Functions & Graphs) that was designed to help colleges place each admitted student in the appropriate mathematics course. The DTMS were administered in freshman level mathematics courses at 36 institutions that represented a broad spectrum of two-year and four-year colleges. A pre-post design was used in seven institutions, with a pretest only in the other colleges. Results indicated that the DTMS were: (1) sensitive to gains from instruction in a single semester; (2) highly correlated with course grades obtained concurrently; (3) predictive of end of course grades when the DTMS were administered at the beginning of the course; (4) a better predictor of course grades than SAT-Mathematical scores for remedial courses; (5) related to course difficulty as perceived by students; (6) capable of demonstrating a trait-treatment interaction in a precalculus-calculus sequence; and (7) related to the content of mathematics courses as judged by mathematics faculty members. (Author/RL)