Trust is an increasingly important concept on the Internet, especially for Electronic Commerce. There are a number of trust- models on the Internet providing authentication which attempt to achieve the maximum of trust with minimum of risks. These include: X.509 standard Public Key Infrastructure (PKI), other PKI such as Pretty Good Privacy (PGP), the Simple Public Key Infrastructure (SPKI) and a Simple Distributed Secure Infrastructure (SDSI). These models use public key encryption techniques, certificates, and digital signatures. A certificate (or Digital ID) is used as a trust-token between different parties on the Internet to tell others you are really who you say you are. This work is a survey of different existing trusted models, their certificates, their structures and ways of handling transitivity of trust, Certificate Revocation Lists (CRL). It includes some practical examples of different trusted- models. This survey looks at their capabilities and strengths, and provides an independent point of view for general users to make their choice of certificate in different environments.