The drug-removal theory holds that enforcement is effective when it captures drugs and removes them from the supply. This theory would make sense only if the supply curve for the marijuana market was vertical or close to vertical (capacity to produce marijuana is limited), and this is not the case. The through-put capacity theory reasons that if enforcement can restrict supply and raise marijuana prices, it can disable one or more large supply organizations. Neither theory nor evidence supports this theory. A third theory holds that enforcement is effective by imposing a cost on drug dealing that impacts the affordability of marijuana use. This in turn discourages persons from using marijuana. Although superior to the other two theories, this theory fails to explain why the price of marijuana is far higher than the risks of the trade can explain. A fourth hybrid theory, the "learning curve" theory, starts with the cost-imposition model but borrows some features from the through-put capacity model. This theory, however, is difficult to test and is less precise in contingent predictions about the results of enforcement changes than is the cost-imposition theory. 9 graphs, 9 notes.