The signaling pathways for converting odorant stimuli from the environment into a signal that is interpreted by the brain has largely been elucidated. The current and formidable challenges lie in understanding the interactions among the second messenger pathways, determining the specificity of olfactory receptors in recognizing chemically similar odorants, and elucidating the regulatory pathways that confine receptor expression to a small subset of the olfactory neurons. It is the sum total of these contributions that result in the ability of the olfactory system to detect odorant with high sensitivity and specificity.