This collection documents the research interests of Rainer Weiss, professor of physics at the Massachusetts Institute of Technology (MIT), who was also affiliated with the Research Laboratory of Electronics, and later the Center for Space Research at MIT. The bulk of materials date from the early 1970s to 1999, and document the experimental and theoretical aspects of the COBE satellite and balloon observations related to cosmic microwave background measurements (Series 1 and 2) as well as materials related to the founding and development of the Laser Interferometer Gravitational-Wave Observatory (LIGO) (Series 3). In addition, the materials help document inter-institutional (university and government) scientific collaborations. Laboratory notebooks in Series 4 are from both areas of research. To date (2016) the collection does not include personal information about the celebrations and announcement of the detection of gravitational waves (in 2015 and 2016).