We study the matching of patent applications to examiners at the U.S. Patent and Trademark Office. Using test statistics originally developed to identify industry agglomeration, we find strong evidence that examiners specialize in particular technologies, even within relatively homogeneous art units. Examiner specialization is more pronounced in the biotechnology and chemistry fields, and less in computers and software. Evidence of specialization becomes weaker, but does not completely disappear, if we condition on technology sub-classes. There is no evidence that certain examiners specialize in applications that have greater importance or broader claims. More specialized examiners have a lower grant rate and produce a larger narrowing of claim-scope during the examination process. We discuss implications for instrumental variables based on examiner characteristics.