This article provides an overview of the study design, the sampling procedure, the applied instruments, and the research potential of BiKS-8-18 “Educational Processes, Competence Development, and Formation of Educational Decisions in Preschool and School Age”). BiKS-8-18 is a panel study, which followed students in the German federal states of Bavaria and Hesse from 2006 through 2016 to trace their educational trajectories and competence developments. One of the major aims of the study was to gather suitable longitudinal data to explain previous findings of international large-scale assessments such as PISA, e.g., on the importance of social origin for educational achievements. The study is characterized by an interdisciplinary approach that includes pedagogical, psychological, and sociological perspectives. The study followed students from primary school over their educational career until their transitions from general education into tertiary education or vocational education when they were around 18–19 years old. During their time in primary school and secondary school, the study assessed children’s competencies and their familial and institutional learning environments. Additionally, children’s parents and teachers were interviewed. The initial sample consisted of 2,395 children in Grade 3. In Grade 5, the sample was expanded by further 879 secondary school students within BiKS-8-18 study classes.