The categorizations which humans make of the concrete world are not arbitrary but highly determined. In taxonomies of concrete objects, there is one level of abstraction at which the most basic category cuts are made. Basic categories are those which carry the most information, possess the highest category cue validity, and are, thus, the most differentiated from one another. The four experiments of Part I defined basic objects by demonstrating that in class inclusion taxonomies of common concrete nouns in English, basic objects are the most inclusive categories whose members: (1) possess significant numbers of attributes in common, (2) are used by means of similar motor programs, (3) possess shapes which are significantly similar, and (4) can be identified from averaged shapes of members of the class. The eight experiments of Part II explored some implications of the structure of categories. Basic objects were shown to be: (1) the most inclusive categories for which a concrete image of the category as a whole could be formed; (2) the first categorizations made during perception of the environment; (3) the earliest categories sorted and earliest named by children; and (4) the categories most codable, most coded, and most necessary in language. Universality and implications of the results were discussed. (Author/CLK)