This paper presents a comprehensive review of the state-of-the-art in research on warehouse operation planning. We first present a unifying framework to classify the research on different but related warehouse problems. Within this framework, historical progress and major results are summarized with an emphasis on how the research on these problems evolved and the relationships between various problems. Future research directions are identified and discussed. The scope of this paper is restricted to warehouse operation planning methods. There are a lot of related results on performance evaluation, which we believe deserve a separate discussion since it is a key issue in warehouse design and operation that provides the basis for intelligent decision making. The companion paper (Gu et al., 2005) provides a detailed discussion on this topic together with warehouse design, computational systems, and case studies. Readers may also refer to Rowenhorst et al. (2000) for a recent survey on the overall warehouse design and operation problems.