How does the neuronal network of the brain mediate its function? This fundamental question in biology is still a puzzle, despite decades of intensive work. A seemingly unsolvable obstacle when studying the brain is its enormous complexity.A powerful approach to understand our brain uses “simple” brains of animal models, in which one can genetically identify and manipulate neurons. The brain of the fruit fly Drosophila melanogaster is widely used as an impacting model system to study how the brain functions with the advantage that the fly brain is much simpler compared to the mammalian brain. Even another one order of magnitude simpler is the brain of the Drosophila larva consisting of an estimated 10’000 neurons. We use the Drosophila larva as an excellent model to genetically dissect a comparably simple visual system. The eye of the larva consists only of two types of photoreceptor neurons: four blue-sensitive cells expressing Rhodopsin5 (Rh5) and eight green-sensitive cells expressing Rhodpsin6 (Rh6). The photoreceptors transmit the information to a small set of target neurons in the larval optic neuropil (LON). We found that only 12-15 brain interneurons connect to the LON, which can be classified in four different groups, according to location and neurotransmitter they use.The goal of this project is to dissect neuronal circuits of visual information processing: from mapping synaptic connections in the circuit to the behavioural impact of neurons. Preliminary results of my laboratory and a “step-by-step strategy” support the feasibility to reach these ambitious goals (Sprecher et al., 2011; Von Essen et al., 2011, Keene et al., 2011).