In Structures in Science, Theo A. F. Kuipers presents a detailed analysis of reductive, including microreductive, explanations. One goal of a microreduction is to explain the laws governing a structured object in terms of laws about its parts, plus a description of its structure. Kuipers refers to structures in his book, and uses the idea of a “structure representation function,” but does not characterize the relevant concept of structure. To characterize microreductions fully, we need an adequate characterization of the relevant sense of “structure.” After discussing examples, I present general analyses of bonds and of structured wholes. My analyses apply from physics to the social sciences, the latter illustrated by a hypothetical robotic social structure. Since Kuipers’ philosophical position appears to be generally compatible with my own, I do not critique of any part of his work. Instead, this article is intended to fill in a gap in his presentation.