Eye-trackers suitable for use with infants are now marketed by several commercial vendors. As eye-trackers become more prevalent in infancy research, there is the potential for users to be unaware of dangers lurking "under the hood" if they assume the eye-tracker introduces no errors in measuring infants' gaze. Moreover, the influx of voluminous data sets from eye-trackers requires users to think hard about what they are measuring and what these measures mean for making inferences about underlying cognitive processes. The present commentary highlights these concerns, both technical and interpretive, and reviews the five articles that comprise this Special Issue.