Reliably answering questions about representation and parliamentary behavior requires data about which parliamentarian was where, and at what time. However, parliament membership is not stable over time. For example, it is common for politicians to change office (we find up to 40% turnover between elections). Consequently, parliament membership, as well as party and party group composition change on a daily basis. To address the challenges that these fluctuations present, we introduce a new open-source database:‘ ‘Parliaments Day-By-Day” (PDBD). PDBD currently contains demographic and day-by-day membership data for the national parliaments of Germany, Switzerland, and the Netherlands, covering the period between 1947 and 2017, and comprising a total of 21 million parliament-legislator-day observations. We demonstrate the usefulness of this high-resolution data in a concise study of the day-by-day development of parliaments in terms of gender and seniority. This reveals hitherto unknown patterns of early turnover, gendered replacement, and seniority.