The aim of this chapter is to illustrate the application of the quasi-simplex model (QSM) for reliability estimation in longitudinal data and to employ it to obtain information about the reliability of the European Union—Survey on Income and Living Conditions (EU-SILC) data collected between 2012 and 2017. Reliability of two survey questions is analysed: one which asks respondents about the financial situation in their households, and one which requests information about respondents’ health. Employing the QSM on the two items resulted in 80 reliability estimates from 17 and 11 European countries, respectively. Results revealed statistically significant differences in reliability between post-communist Central and Eastern European (CEE) countries and the rest of Europe, and similar patterns of the size of reliability estimates were observed for both items. The highest reliability (i.e. reliability over 0.8) was observed in CEE countries such as Bulgaria, Romania, Czechia, Poland, and Hungary. The lowest reliability (i.e. reliability lower than 0.7) was observed for data from Sweden, Slovenia, Norway, Spain, Portugal, Austria, Italy, and the Netherlands. The remarkable variation in longitudinal reliability across culturally and historically different European regions is discussed both from substantive and methodological perspectives.