This paper reviews the psychological theories and research which deal with the meaning of words in the course of human psychological development. The three major premises of the review are: (1) that word meaning is conducive to empirical study, (2) that rules can be produced that apply to most word meanings, and (3) that human development has implications for the qualities shared by human word meanings. Four psychological approaches to word meaning are identified, discussed, and compared: behaviorist, signal-systems, cognitive, and linguistic. The various theories and empirical research findings which support each approach are examined in detail; common elements are summarized. Implications for research are suggested, and implications for early education (prereading), later education (including reading), and testing are considered for each approach. (Author/ED)