We model competitive bundling and tying, allowing for marginal cost savings from bundling, fixed costs of product offerings, and variation in customer preferences. Pure bundling can arise either because few people demand only one component or because, with high fixed costs, a single product efficiently satisfies customers with diverse tastes. We conclude by analyzing empirically the bundling of pain relievers with decongestants. The discount for the bundled product is large. We argue that our model provides a simpler, more compelling explanation for the size of the discount than the demand-centered approach to bundling by a monopolist.