This paper, presented at the 2001 Physics Education Research Conference, discusses the physics education research group at Rensselaer which is working to develop an assessment tool that will measure the problem-solving ability of introductory physics students. In its final form, the tool will consists of approximately 30-40 multiple-choice questions related to a limited number of classical mechanics topics. There are currently four types of questions included in the exam: attitudinal questions, quantitative problems that require students to identify the underlying principles used in solving the problem but not an explicit solution, questions that ask students to compare posed problems in terms of solution method, and quantitative problems requiring solution. Although the assessment is still under development, we have performed preliminary validation studies on questions requiring students to identify underlying principles. Specifically, both an ANOVA and a Fisher LSD test have been preformed. These evaluations showed (at the 98% and 95% confidence level, respectively) that wrong answers on assessment questions correlate to below average performance on the problem solving portion of the final course exam.