Calcium is one of the most important signalling molecules in plants. Cellular changes in calcium concentration act as a regulator in many growth and developmental processes as well as in response to a diverse set of biotic and abiotic stimuli. Calcium signals are decoded by an array of cellular sensor proteins, which mediate the transformation of the calcium signal into an appropriate cellular response. In plants, a significant diversity and abundance of calcium sensor proteins has evolved, most likely to contribute to the specificity and flexibility of this signalling network. A good example for this expansion is calmodulin (CaM), a ubiquitous calcium sensor in all eukaryotic organisms that plays a crucial role in many cellular processes. Like most eukaryotes, plants contain a small set of canonical CaMs. Additionally, plants exclusively possess a group of calmodulin-like proteins (CMLs), of which the Arabidopsis genome encodes over 50 isoforms. This extreme diversity of CaMs and CMLs indicates a significant expansion of this type of regulation in plants compared to other organisms. Despite their obvious importance, the physiological function of most CaMs and CMLs remains largely elusive. In this review we present recent insights into research on CaMs and CMLs, their specific role in calcium regulation and potential subcellular localisation.