Publisher Summary This chapter focuses on the concept of trust in a Windows Server 2003 public key infrastructure (PKI). It explores Windows Server 2003 PKI trust types and trust models, and it looks at how PKI trust relationships can be defined and managed in a Windows Server 2003 environment. Trust is a very complex notion that cannot be easily defined or classified. Trust relationships can be distinguished into three basic types: direct, third-party, and extended third-party trust relationships. Both third-party and extended third-party trust relationships are also referred to as indirect trust relationships. In a direct or peer-to-peer trust relationship, PKI trust models provide a technological framework for the management of PKI trust relationships among certificate authorities (CAs) and between CAs and PKI users. The chapter discusses different types of PKI trust models such as the hierarchical, networked, meshed, bridge CA, and hybrid trust models. The chapter also shows the richness of PKI when it comes to trust definition—no other security protocol or technology available today can define trust in such a granular way.