In the last decade, several different optical imaging techniques, either based on various voltage or calcium dyes, or more recently on modified fluorescent or bioluminescent proteins (genetically encoded) that are sensitive to calcium, have been developed to study neuronal activity, and especially groups of neurons, with the goal of mapping and deciphering the neural code underlying major neurophysiological functions. Genetically Encoded Functional Indicators brings together expert contributors to present the development of recent genetic techniques that allow for generating genetically encoded activity sensors in order to investigate neuronal activity. Each chapter describes a specific sensor and its utilization to study neuronal activity in a particular way.