This study examines the role of information uncertainty (IU) in predicting cross-sectional stock returns. We define IU in terms of "value ambiguity", or the precision with which firm value can be estimated by knowledgeable investors at reasonable cost. Using several different proxies for IU, we show that: (1) On average, High IU firms earn lower future resturns (the "mean" effect), and (2) Price and earnings momentum effects are much stronger among high IU firms (the "interaction" effect). These findings are consistent with theoretical models that feature investor overconfidence (Daniel et al. (1998)) and information cascades (Bikhchandani et al. (1992)). Specifically, our evidence indicates that high IU exacerbates investor overconfidence and limits rational arbitrage.