Since the start of the new millennium, international comparative large-scale studies have become one of the most well-known areas in the field of education. However, the International Association for the Evaluation of Educational Achievement (IEA) has already been conducting international comparative studies for about half a century. The present study aims to demonstrate how to link recent and older studies onto the same scale in order to study long-term trends within and across countries. It discusses the comparability of the assessment material in the Progress in International Reading Literacy Study (PIRLS) and previous IEA studies on reading at the end of primary school. Thereafter, we use a concurrent calibration of all item parameters to link the studies onto a common IRT scale extending from 1970 until the present.