Omalizumab (Xolair), a monoclonal antibody, is a new agent that blocks the allergic cascade at its primary step. This drug offers substantial promise for patients with moderate-to-severe, persistent allergic asthma that is not well controlled. But due to the cost of the drug, limitations on dosage, and available clinical trial data, it is not a first-line therapy. This review discusses how this drug works, which patients will be candidates for it, and the practical aspects of using it.