The German Observational Study of Adult Twins (GOSAT) is the largest population-based observational twin study in Germany to date. Embedded in the Bielefeld Longitudinal Study of Adult Twins (BiLSAT), it addresses the etiology of personality, temperament and cognitive ability in a sample of 300 monozygotic (MZ) and dizygotic (DZ) adult twin pairs between 18 and 70 years of age. A major aim of the GOSAT lies in the utilization of different modes of measurement, (i.e., peer reports and observational data), in addition to self-reports which have been used predominantly in past behavioral genetic research on personality and temperament in adults. Participants completed a full day assessment at the University of Bielefeld including videotaped social interactions and presentations, psychometric intelligence tests and computerized elementary cognitive tasks as well as objective measures and unobtrusive behavior counts. The research design of the GOSAT was devised to reduce the potential impact of systematic rater bias on estimates of genetic and environmental influences to a minimum. In combination with extensive self- and peer report data on key personality and personality related dimensions available from the BiLSAT, the GOSAT provides a rich dataset, which currently includes DNA samples from 80% of its participants.