Numerous drugs have the ability to alter our perception, cognition, and mood. Some of these compounds, such as ketamine and serotonergic psychedelics, have also shown promise as treatment for mental illnesses. The behavioral effects are often long-lasting, presumably because the drugs act on synapses and dendrites to induce plasticity in the brain. In this talk, I will describe a series of studies from my lab aimed at understanding the mechanism of action of psilocybin, using subcellular-resolution two-photon imaging, in vivo electrophysiology, rabies viral tracing, and other molecular and behavioral approaches in mice. The results provide insights into the drug action of psychedelics on neural circuits.