Until 2006, BKPyV and JCPyV were the only known human polyomaviruses. A third polyomavirus, simian virus 40 whose natural host is the macaque was accidently introduced into man because of contaminated poliovirus vaccines, although there is epidemiological evidence that SV40 may be transmitted between man independently from contaminated vaccines. Since 2007, 10 new human polyomaviruses have been identified: KIPyV, WUPyV, Merkel cell polyomavirus, trichodysplasia spinulosa-associated polyomavirus, and human polyomaviruses 6, 7, 9, 10, STL, and 12. Moreover, the DNA of the monkey lymphotropic polyomavirus has been amplified from human peripheral blood. Seroepidemiological studies frequently based on the presence of antibodies against the major capsid protein VP1 or virus-like particles indicate that most human adults have been exposed to many, if not all, human polyomaviruses. However, because of the high amino acid sequence identity between VP1 of some human polyomaviruses, cross-reactivity of antibodies is occasionally observed. In addition, human sera possess reactivity against VP1 of polyomaviruses from other species, suggesting serological cross-reaction with known or closely related, yet unidentified human polyomaviruses and/or the possibility of zoonotic transmission. Thus, current serological results should be interpreted with caution, and controls excluding cross-reactivity with other polyomaviruses are required. Copyright © 2013 John Wiley & Sons, Ltd.