Polyclonal antibodies have a century-old history of being effective against some viruses; recently, monoclonal antibodies (mAbs) have also shown success. The humanized mAb Synagis (palivizumab) remains still the only mAb against respiratory syncytial virus (RSV) infections approved by the U.S. Food and Drug Administration (FDA). Recently, several potent human monoclonal antibodies (hmAbs) targeting the Severe Acute Respiratory Syndrome-Associated coronavirus (SARS CoV) S glycoproteins were developed quickly after the virus was identified in 2003. Among these antibodies, m396 and S230.15 exhibit exceptional potency and cross-reactivity as they neutralize isolates from the first and second outbreaks and from palm civets both in vitroand in mice. Similarly, the first fully hmAbs against two other paramyxoviruses, Hendra virus (HeV) and Nipah virus (NiV), which can cause up to 75% mortality, were recently developed; one of them, m102.4, shows exceptional cross-reactive potency against both NiV and HeV. Three-dimensional molecular structures of envelope glycoproteins from these viruses in complexes with antibodies and/or receptors were recently determined. Structural analyses along with other experiments have provided insights into the molecular mechanisms of receptor recognition and antibody neutralization, and suggested that these antibodies alone or in combination could successfully fight the viruses’ heterogeneity and mutability which is a major problem in the development of effective therapeutic agents against viruses, including therapeutic antibodies.