Abstract: Adult development is becoming an important field of study for psychology and other disciplines. Little has been done, however, to conceptualize the nature of adult development and to define the major issues in this field. The author summarizes his own formulations of life course, life cycle, life structure, and the adult development of the life structure in early and middle adulthood. He then discusses six major issues that must be dealt with by every structural approach to adult development: What are the alternative ways of defining a structural stage or period? What relative emphasis is given to the structures as compared to the transitional periods? How can we make best use of the distinction between hierarchical levels and seasons of development? Are there age-linked developmental periods in adulthood? What are the relative merits and limitations of various research methods? How can we bring together the developmental perspective and the socialization perspective? The study of adult development is, one might say, in its infancy. It has been taken seriously in the human sciences for only the past 30 years or so, largely under the impact of Erikson's (1950, 1958, 1969) germinal writings. Erikson's most obvious contribution was his theory of stages in ego development. What is less obvious is that his view of development is deeply grounded in his conceptions of the life cycle and the life course. Each ego stage has its primacy at a particular age level or segment of the life cycle, from infancy to old age. The sequence of age segments and ego stages thus provides a representation of the life cycle as a whole; the meaning of a stage is defined in part by its place in the total sequence. In addition, his developmental concepts arose out of his primary concern with the individual life course: the process of living, the idea of life history rather than case history, the use of biography rather than therapy or testing as his chief research method. Without abandoning the distinction between self (psyche, personality, inner world) and external world (society, culture, institutions, history), he gave first consideration to the life course--the engagement of self with world. Although a good deal has been learned since the 1950s about specific features of adult life, very little has been done to advance the general theory of adult development. At the same time, various fields of psychology (such as child development, gerontology, personality, social, clinical, and counseling psychology), as well as the social sciences and humanities, are becoming more aware that they need--and lack--an adult development perspective. Adult development is, in short, a significant problem for psychology as a discipline and an important link between psychology and other disciplines, including sociology, biology, and history. I have two primary aims here. First, I will present my conception of adulthood and of a developmental process within it. My intention is to explicate a theoretical position, not to prove it nor to argue for its superiority over others. The theory originated in my initial study of men's lives (Levinson, 1977, 1978). It has evolved over the last few years, particularly through my current research on women's lives (Levinson, in press). It is supported by a number of other studies (e.g., Gooden, 1980; Holt, 1980; Kellerman, 1975; Levinson, 1984; Stewart, 1976), but a great deal must yet be done to test and modify it. The theory includes the following elements: (a) The concepts of life course and life cycle, which provide an essential framework for the field of adult development; within this framework, studies of one process or age level can be connected to others, but without it, we have a miscellany of findings and no integrated domain of inquiry; (b) the concept of the individual life structure, which includes many aspects of personality and of the external world but is not identical with any of these and evolves in its own distinctive way; and (c) a conception of adult development--the evolution of the life structure in early and middle adulthood. Life structure development is different from, and should not be confused with, the development of personality, social roles, or other commonly studied processes. Second, I will discuss adult development as a field of study. I will consider six major issues that help to define what the field is about and what work must be done to establish it more securely. The list is not complete, but it provides a useful starting point. Reference will be made to the work of others, but the main goal is to clarify my own position. Let it be clear that my aim is not to give a comprehensive review of the work in this field nor to seek consensus among the disparate approaches. I hope that others will be stimulated to present contrasting views.