This paper investigates whether patent applicants strategically withhold citations to material prior art. Citation data suggest that applicants withhold between 21% and 33% of relevant citations. Variation in withholdings is explained by patent portfolio size and indicators of patent value. These data also highlight important differences across technology classes. In particular, firms are significantly less likely to withhold a citation when applying for chemical and drug patents. Robustness checks confirm that these results are not explained by inventor and attorney familiarity with citations, patent examiner heterogeneity, or the nationality of the inventor.