This chapter presents an overview on the essential findings from research based on the neurophysiology of animal fear learning. Some behavioral studies have shown that conscious awareness in healthy participants may be necessary for fear conditioning. Behavioral studies manipulating conscious awareness of conditioned stimulus (CS)-unconditioned stimulus (US) contingencies in healthy adults often employ backward masking procedures, wherein the CS is rapidly presented and then quickly "masked" by another stimulus. The study of human fear conditioning has typically involved laboratory or neuroimag-ing examinations that make use of two-dimensional objects as CSs presented to the subject over a computer monitor. This chapter highlights behavioral and neuroimaging research on human fear learning that has been informed and supported by advancements in nonhuman animal research. A widespread distribution of efferent projections from the amygdala is important for modulating information processing broadly across the brain.