The antennal lobe (AL) of an insect is the functional analog of the olfactory bulb in mammals. The first-level synaptic interaction of large numbers of multiple types of olfactory receptor neurons (OSNs) with AL interneurons serves the function of reliably coding a vast range of odorants and their mixtures and the separation between odor identity and odor concentration. Honeybees learn and discriminate among a seemingly unlimited number of odors (natural and artificial), categorize odor mixtures as unique stimuli, identify odors within 250 ms and odor sequences within 6 ms, and generalize odors according to the respective combinatorial glomerulus activity patterns in the AL. Therefore, the AL is the first-order neuropil serving basic functions of odor discrimination, categorization, generalization, and learning. This chapter first describes the internal organization of the AL, its inputs, and its outputs, and then analyzes the local circuit of a prototypical glomerulus.