I analyze the marketing strategy of an incumbent monopolist facing a threat of entry. Product quality is unknown to consumers, and the monopolist’s cost is unknown to the potential entrant. The incumbent uses both price and advertising to signal cost and quality. The monopolist faces a dilemma because signaling a high quality attracts customers but requires a high price, whereas signaling low cost prevents entry but requires a low price. I characterize the unique (stable) separating equilibrium and show that dissipative advertising may be used, while it is never used if either quality or cost is known. Some equilibria may involve pooling on cost. A welfare analysis indicates that potential entry may improve welfare and that the effect of unknown quality is not always negative when it interferes with entry deterrence.