Spiders are among the world’s largest and most diverse animal lineages. They have diverse behaviors. For example, some families build webs for prey capture whereas others are active hunters that stalk their prey using vision. These behaviors correlate with differences in their visual systems. The number, complexity and arrangement of spider eyes vary across spider families. Spiders thus provide an opportunity to undertake comparative studies. In particular, very little is known about the neurobiology of spider brains. Spider brains are very different than those of insects and even other chelicerates, such as scorpions and harvestmen. Due to the absence of a dedicated olfactory/chemosensory appendange, spiders lack the associated neural processing regions found in most other arthropods. Spiders are also the only known chelicerates that lack immunoreactivity to proteins involved in arthropod learning and memory found in other arthropods. Instead, most of a spider’s protocerebrum, the first segment of its fused nervous system, is dedicated to visual processing. The size and organization of the visual processing regions of the protocerebrum varies across spider families.