All human co-operation is based on trust, meaning that we choose co-operation partners and make commitment decisions based on how much we trust the other party. Digital certificates and public-key infrastructures represent an attempt to mimic real-world human assessment of identity and trustworthiness in an automated and mechanical fashion, but present implementations are based on a very limited trust model making them inadequate as a general tool for trust assessment and decision making. This paper describes public-key infrastructures in general and discusses issues related to trust management of public-key infrastructures.