Sensory systems are the input pathways to the nervous system and can be windows into its working. The responses of neurons or individuals to controlled sensory stimuli enable us to relate the known (sensory stimuli) to the unknown (the nervous system response). Brain pathways and neuronal mechanisms related to sensory signals are often better understood than most pathways and mechanisms within the brain. This means that brain slices or single cells from sensory systems make good experimental preparations for more detailed molecular studies. We already have some idea of what these cells and circuits do - but how do they do it?