Neutralization is the ability of antibody to bind to and inactivate virus infectivity under defined conditions in vitro. Most neutralizing antibodies also protect animals in vivo, but protection is more complex as it also involves interaction of antibody with cells and molecules of the innate immune system. Neutralization by antibody can be mediated by a number of different mechanisms: by aggregation of virions, destabilization of the virion structure, inhibition of virion attachment to target cells, inhibition of the fusion of the virion lipid membrane with the membrane of the host cell, inhibition of the entry of the genome of non-enveloped viruses into the cell cytoplasm, inhibition of a function of the virion core through a signal transduced by an antibody, transcytosing IgA, and binding to nascent virions to block their budding or release from the cell surface. The mechanism of neutralization is determined by the properties of both a virion epitope and the antibody that reacts with it. Further, since a virus has at least several unique epitopes sited in different locations on the virion, and since the paratope and other properties of the reacting antibody can vary, this means that a virus can be neutralized by several different mechanisms. Understanding the processes of neutralization informs the creation of modern vaccines, and gives valuable insights into virus-cell interactions.