The complexity and rapid expansion of the entire field of psychology make it appropriate to speak of "psychologies" when acknowledging the need for specialization of training and expertise. Nevertheless, unifying trends (UTs) exist in psychology, even though there can be no single set of theoretical principles to account for all empirical findings. UTs can extend in various directions, such as across theoretical orientations, across major fields of specialization, across methodologies, across cultures, across age levels, between theory and empirical research, and between basic science and practice. For example, the following UTs are currently observable in the field of psychometrics: recognition of psychological trait hierarchies; research on trait formation; the study of interactions of traits and situations; linkage between cognitive and affective variables; the study of the contributions of heredity and environment; the study of psychological testing in context; and contributions of cognitive psychology. UTs are also apparent in other areas of psychology; two examples are the evolving linkage between the experimental psychology of learning and the educational psychology of instruction, and an integrative movement in psychotherapy. Some conspicuous instances of the reverse trend are evident, in which new developments are treated as revolutionary. In such an approach, existing ideas, theories, or methods are discarded to make room for new ones. Such a strategy may yield short-term benefits for the individual psychologist, but it tends to retard the progress of psychology as a science, its effectiveness in solving practical problems, and its acceptance and credibility by the public. A 116-item list of references is included. (SLD)