The purpose of this paper is to review evaluation theories and techniques in both the medical and educational fields and to propose an evaluation theory to explain the condition variables, the method variables, and the outcome variables of student assessment in a problem-based learning (PBL) approach. The PBL definition and process are presented, and the following goals of PBL are summarized: metacognition; self-directed learning; critical thinking and problem solving skills; knowledge acquisition, retention, and use; collaborative learning; and higher motivation and positive attitude. A categorization scheme of assessment is presented that includes a wide range of evaluation techniques and tools to assess students' behaviors and products in a PBL approach. Process-oriented and outcome-oriented approaches are used to classify the assessment methods. Process-oriented evaluation methods include: tutor, peer ratings; self-evaluation; unobtrusive measure; oral examination; observation of a patient interview and examination; patient problem simulations; review of case record; authentic assessment; and performance assessment. Outcome-oriented evaluation methods include: review of case record; student-judged evaluation; multiple-choice examinations; short-answer or word-completion examinations; essay examinations; patient problem simulations; and portfolio assessment. (Contains 25 references.) (MES)