Educators are now focusing on the improvement of reasoning skills. A key to success in improving such skills will be the ability of the individual teacher to measure skills in a valid and reliable manner. This guide will help teachers acquire the needed assessment skills. Major topics include measuring thinking skills, learning to plan an assessment, and making assessment work in the classroom. Five workable categories of thinking skills are defined: (1) recall; (2) analysis; (3) comparison; (4) inferences; and (5) evaluation. The ways in which these are related to three common forms of assessment--oral questions, paper-and-pencil tests, and performance tests--are discussed. A basic assessment planning chart is suggested. Examples of completed and partially-completed assessment charts are given for all five skill categories and three forms of assessment in the areas of elementary English, junior high school science and social studies, and high school social studies. Guidelines are then provided for daily use in the classroom. Practice and knowledge of the basic skills of sound test item writing and performance assessment are essential to assessing reasoning skills. Resources for classroom assessment, answers to a sample progress checklist, and cognitive domain levels according to B. S. Bloom et al (1956) are appended. (SLD)