One fundamental question in neuroscience is how the brain processes and stores information. As the information-processing elements in the brain, neurons communicate via specialized connections called synapses. The majority of excitatory synapses reside at dendritic spines, which serve as a good proxy for synaptic connectivity. Using transcranial two-photon microscopy to visualize fluorescently-labeled neurons in transgenic mice, our recent studies followed the dynamics of spines on apical dendrites of L5 pyramidal neurons in the living brain. In this talk, I will discuss our findings on the spatiotemporal changes of spine dynamics during motor-skill learning. I will also present our new data exploring the molecular/cellular mechanisms underlying activity-dependent spine pruning during development.