The idea to reduce the self-weight of concrete slabs by putting voids in the centre of the cross-section, dates to the beginning of the previous century. Several inventors from different countries applied for patents on various systems. The present article is mainly based on an analysis of patents published during the first half of the 20th century, and personal experiences from 1960 on. Patents usually offer a complex description of inventions (claims). Reconstructing the history of hollow core slabs based on these patents is a laborious but fascinating exercise. This article aims to give a general overview and is not meant to be exhaustive.