Two-photon microscopy is a powerful imaging tool for scattering samples such as the brain. In combination with fluorescent indicators and a chronic cranial window it allows to image neuronal activity in the brain. Additionally, it can be performed in awake animals and combined with behavioral tasks. In my talk I will present the basic principles of two-photon microscopy and introduce the projects of my lab in behaving mice. I will present Ca2+ imaging of cerebral neurons in posterior parietal cortex, Ca2+ imaging of layer 6 neurons in visual cortex, and imaging of Ca2+ hotspot activity in astrocytes of barrel cortex. I will also present simultaneous voltage and Ca2+ imaging of cerebellar Purkinje neurons.