Midlife in the United States (MIDUS) is a national longitudinal study of health and well-being (http://midus.wisc.edu/). It was conceived by a multidisciplinary team of scholars interested in understanding aging as an integrated bio-psycho-social process, and as such it includes data collected in a wide array of research protocols using a variety of survey and non-survey instruments. The data captured by these different protocols (comprising around 20,000 variables) represent survey measures, cognitive assessments, daily stress diaries, clinical, biomarker and neuroscience data which are contained in separate flat or stacked data files with a common ID system that allows easy data merges among them. All MIDUS datasets and documentation are archived at the ICPSR (http://www.icpsr.umich.edu/) repository at the University of Michigan and are publicly available in a variety of formats and statistical packages. Special attention is given to providing clear user-friendly documentation; the study has embraced the Data Documentation Initiative (DDI) metadata standard and produces DDI-Lifecycle compliant codebooks. Potential for secondary use of MIDUS is high and actively encouraged. The study has become very popular with the research public as measured by data downloads and citation counts (see Reuse Potential below).