The enigmatic arabinogalactan-proteins (AGPs) have fascinated and challenged researchers for decades. In the 1960s, AGPs were being readily extracted from a large number of species due to their water solubility. At the time, research was focused on the carbohydrate component and the existence of protein core was largely unknown. The association of glycans with hydroxyproline-containing proteins was alluded to as early as 1965, and nearly 10 years later an arabinogalactan-peptide from wheat was isolated that conclusively showed the covalent association of protein and glycans. A further 50 years of research has provided insight into the diversity of the protein backbones and glycan structures; their presence across evolutionary ‘time’ and the ‘space’ they occupy at the plasma membrane-cell wall interface that, combined with tissue specificity, can have important signalling functions. This article highlights recent developments that are enabling insights into the evolution, biological roles, and molecular mechanisms of this diverse family.