The connections of the amygdala in rats, cats, and monkeys can be divided into three systems: (1) A largely forebrain system provides sensory information to the amygdala from the olfactory cortex, ascending taste/visceral pathways, posterior thalamus and sensory association cortex. Most of these connections are reciprocal and also support amygdaloid modulation of sensory processing. (2) A set of projections extends from the amygdala to the hypothalamus and brain stem, which appears to modulate visceral function in relation to emotional stimuli. (3) A forebrain circuit involves the amygdala with the ventromedial frontal, rostral insular, and rostral temporal cortical areas, the medial thalamus and ventromedial basal ganglia, and seems to be directly involved in emotional behavior and mood. All of these systems are present in all mammals studied; the major differences lie in the greater elaboration of the cerebral cortex in primates.