The Midlife Development in the United States (MIDUS) Series is a data collection stemming from the work of the John D. and Catherine T. MacArthur Foundation Research Network on Successful Midlife Development (MIDMAC). MIDMAC is an interdisciplinary research group consisting of numerous scholars from a wide range of disciplines and backgrounds. It was established in 1989 to study a little known period in the lifespan -- middle age. Midlife, the years between 30 and 70, is perhaps the least studied and most ill-defined of any period of life. It abounds with changing images and myths, such as the "midlife crisis," the "change of life," the "empty nest syndrome," and many more. However, there has been little documentation about what really happens, biologically and psychologically, during this extended period of time. The primary objective of MIDMAC is to identify the major biomedical, psychological, and social factors that permit some people to achieve good health, psychological well-being, and social responsiblity during their adult years. To do this, MIDMAC collected a series of data to establish an empirical basis for documenting what really happens in the middle years and to identify the factors that determine the course of midlife development. The first wave of data collection (MIDUS I) began in 1995 with a National Survey of Midlife Development in the United States. The main data collection consisted of a general population survey, as well as surveys of siblings of the general population respondents, and a twin pairs sample. The main data collection also included an oversample of five metropolitan areas. In addition to the main national survey, random subsamples of respondents were recruited to participate in in-depth investigations of selected topics, such as management strategies for coping with stressful experiences, a national study of daily experiences, and a study of psychological experiences. In 2002 the University of Wisconsin Institute on Aging was awarded a grant from the National Institutte on Aging to continue the MIDUS series. The MIDUS II project was designed to collect a second wave of data on the same respondents approximately 10 years later.