Calcium (Ca²⁺) is a ubiquitous intracellular messenger, controlling a diverse range of cellular processes, such as gene transcription, muscle contraction and cell proliferation. The ability of a simple ion such as Ca²⁺ to play a pivotal role in cell biology results from the facility that cells have to shape Ca²⁺ signals in space, time and amplitude. To generate and interpret the variety of observed Ca²⁺ signals, different cell types employ components selected from a Ca²⁺ signalling ‘toolkit’, which comprises an array of homeostatic and sensory mechanisms. By mixing and matching components from the toolkit, cells can obtain Ca²⁺ signals that suit their physiology. Recent studies have demonstrated the importance of local Ca²⁺ signals in defining the specificity of the interaction of Ca²⁺ with its targets. Furthermore, local Ca²⁺ signals are the triggers and building blocks for larger global signals that propagate throughout cells.