The material in this text was compiled to serve as a study guide for a REVIEW of microbiology and immunology suitable for preparing for Step 1 of the United States Medical Licensing Examination (USMLE). I have assumed that you, the reader, have already had a full fledged course covering this discipline. In-depth presentation of material will not be found in this review. You are urged to con­ sult other study aids (lecture notes, textbooks, etc.) for detailed explanations of material that you find troublesome. In general, the text of the book is on the left side of each page; questions, illustrations, summary sentences or phrases, and other study aids are on the right. This format has the intent of getting you involved in the review process. Use a highlighter, put boxes around key statements, answer the questions, and fill in the blanks as you work through the book. Your reward will be proportional to your effort (i.e., no pain, no gain). As the format is a work book, you benefit most by active participation. Write key words in the margins; add to the Review State­ ments by writing out your own; use whatever study skills you have found help you "master" an area.