This paper uses a real options perspective to augment a standard research and development (R&D) investment model and implement a firm-level empirical analysis to assess the practical significance of market uncertainty and its interactions with strategic rivalry and firm size. We use a measure of firm-relevant market uncertainty along with panel data and find that firms invest less in current R&D as uncertainty about market returns increases. The effect of firm-specific uncertainty on R&D investment is smaller in markets where strategic rivalry is likely to be more intense. Furthermore, holding access to financing constant, the effect of uncertainty on R&D investment is attenuated for large firms. Copyright © 2012 John Wiley & Sons, Ltd.