High drop-out rates are considered a major shortcoming of web surveys and considerably threaten data quality if drop-out is systematic rather than random. However, despite growing scholarly attention the knowledge on survey drop-out is still fractional. Previous research mainly addresses the impact of survey design, question wording, and characteristics of the respondents on survey drop-out via ex-post statistical methods. The research presented here is innovative in that the respondents are asked directly about the reasons for dropping out, the interview situation, psychological predispositions, as well as a reduced number of questions on politics and political attitudes in a series of follow-up surveys conducted subsequently to three consecutive online surveys of the GLES. These follow-up surveys, featuring more than 300 interviews with drop-outs, allow for an enhanced understanding of the complex processes underlying the phenomenon. Since the most essential items are also included in the preceding online surveys, the design allows for comparisons between drop-outs and complete responders in order to analyse how both groups differ in terms of their responses to substantive questions. In this regard, our research will add to the knowledge on which respondents are especially prone to drop-out, their reasons for dropping-out, and how these respondents relate to key variables of the respective survey, thus amending the theoretical explanations of drop-out.