In growth models that incorporate public services, the optimal tax policy hinges on the characteristics of the services. If the public services are publicly-provided private goods, that are rival and excludable, or publicly- provided public goods, that are non-rival and non-excludable, lump-sum taxation is superior to income taxation. Many types of public goods are subject to congestion, however, and are therefore rival but to some extent non-excludable. In these cases, income taxation works approximately as a user fee and can therefore be superior to lump-sum taxation. We argue that the congestion model applies to a wide array of public expenditures.