This manual is designed to provide teachers with technically sound, easy-to-follow procedures for developing diagnostic tests to be used with their own students. The definition of diagnostic testing is broadened to include any tests systematically designed to provide information about skills that students have or have not mastered. A five-step test development process divides the manual into five sections: (1) Specify the Diagnostic Skills to be Tested; (2) Create the Test Description; (3) Write the Test Items; (4) Review Test Items; and (5) Try Out Items. Much space is devoted to the conceptual plan underlying the diagnostic test, the skill map. Several strategies for developing the skill map through task analysis are mentioned. The application of research-based analytic methods to the description of important skills is treated as the core of the description section. Once the conceptual base for the diagnostic test is documented in the skill map and the test description, the steps for writing, reviewing, and trying out diagnostic items remain much the same as those used in survey testing programs. Appendices include model test descriptions, many with graphics, a test description review form, sample constructed response scoring guidelines and sample test item formats. (LMO)