Microeconomic principles courses focus on perfectly competitive markets far more than other market structures. The authors examine five possible reasons for this but find none of them sufficiently compelling. They conclude that textbook authors should place more emphasis on how economists select appropriate models and test models' predictions against the empirical evidence. This requires a more even treatment of market structures and less emphasis on perfect competition. Greater emphasis on imperfect competition would also allow textbook authors to address the neglected topic of dynamic efficiency. Principles Textbooks Surveyed are appended. (Contains 1 table and 24 endnotes.)