The Plant Structure Ontology (PSO) is a controlled vocabulary of anatomy and morphology of a generic flowering plant, developed by the Plant Ontology Consortium (POC) The main goal of the POC was to reduce the problem of heterogeneity of terminology used to describe comparable object types in plant genomic databases. PSO provides standardized set of terms describing anatomical and morphological structures pertinent to flowering plants during their normal course of development. Created as a tool for annotation of gene expression patterns and description of phenotypes across angiosperms, PSO is intended for plant genomics databases and broad plant genomic research community. Currently, this ontology encompasses diverse angiosperm taxa; further development will include new model organisms and important crop species. This chapter describes the rationales for creating PSO and discusses the guiding principles for its development and maintenance. The content of the PSO and the ontology browsing functionalities are outlined. The PSO can be browsed and downloaded at www.plantontology.org.