The specificity of connections in the nervous system is essential to translate electrical activity into meaningful neuronal codes. We are interested in understanding how an ensemble of neurons wire together to form specific circuits and how these circuits process information. We address these questions in the olfactory system. Combining real-time imaging, quantitative anatomy, behaviour and electrophysiology we seek to understand the mechanism underlying the topographic organization of the olfactory bulb, in particular the role of the odorant receptor and spontaneous activity. We are also studying adult neurogenesis in the olfactory system, an extreme form of neuronal network plasticity. I will summarize results we recently obtained on these topics.