Uncaging means the activation of biochemically masked (‘caged’) molecules via photolysis, which mimics the physiological release of bioactive compounds. This technique is widely used in neuroscience, where the bioactive molecule is usually glutamate or another neurotransmitter. Using two-photon photostimulation with ultrafast pulsed IR laser light, very precise release of these compounds can be elicited in extremely small volumes. Two-photon imaging is a powerful opportunity to follow the changes evoked in dendrites or spines, even the distribution of receptors on the neurons can be investigated.