Astrocytes were long referred to as passive cells in the central nervous system (CNS). However, the application of fluorescent Ca2+ indicators revealed their "Ca2+ excitability" in response to a variety of stimuli. Since then, evidence showing that astrocytes release chemical transmitters and can modulate synapses has led to a new integrative view of the astrocyte. While it is inarguable that astrocytes play essential roles in nervous system function, and that gliotransmission modulates synapses, a significant challenge lies in developing the appropriate experimental strategy to allow unequivocal identification of roles for gliotransmission in nervous system function.