We develop a model in which the value of a firm’s reputation for quality increases gradually over time. In our model, a firm’s ability to deliver high quality at any given period depends on how much it invests in quality. This investment is the firm’s private information. Also, a firm’s current quality is unobservable. Thus the only observable is a firm’s past performance - the realized quality of the products it delivered. We assume that information about a firm’s past performance diffuses only gradually in the market. Thus, the longer a firm has been delivering high-quality products, the larger the number of potential customers, which are aware of it. We show that in equilibrium, the firm’s investment in quality increases over time, as its reputation - the number of consumers who are aware of its history - increases. This is because the greater its reputation, the more it has to lose from tarnishing it by under-investing and, conversely, the more it has to gain from maintaining it. This is recognized by rational consumers. Therefore, older - and hence larger firms - command higher prices as quality premia. This in turn feeds back into firms’ investment incentives: the fact that they are able to command higher prices motivates older and larger firms to invest and larger a firm is, the more valuable an asset its reputation is.