This article uses data from a large database of voluntarily reported errors to characterize errors in laboratory medicine. Most errors occurred at the preanalytic phase (before the specimen arrived in the laboratory), with many errors arising from misidentification of specimens. The vast majority of the more than 30,000 errors analyzed did not lead to patient harm. A pathology misidentification error that resulted in disclosure of an incorrect diagnosis to a patient is discussed in this AHRQ WebM&M commentary.