A major goal of modern neuroscience is to understand how ensembles of neurons participate in neural circuits underlying behavior. The recent explosion of genetically-encoded circuit analysis tools has allowed neuroscientists to characterize molecularly-defined neuronal types with unprecedented detail. However, since neurons defined by molecular expression can be functionally heterogeneous, targeting circuit analysis tools to neurons based on their activity is critical to elucidating the neural basis of behavior. Here we review genetic strategies to access activated neurons and characterize their functional properties, molecular profiles, connectivity, and causal roles in sensory-coding, memory, and valence-encoding. We also discuss future possibilities for improving these strategies and using them to screen brain-wide activity patterns underlying adaptive and maladaptive behaviors.