Abstract. An electronic signature is considered to be valid, if the signature is mathematically correct and if the signer's public key is classified as authentic. While the first property is easy to decide, the authenticity of the signer's public key depends on the underlying validity model. To our knowledge there are three different validity models described in various public documents or standards. However, up to now a formal description of these models is missing. It is therefore a first aim of the paper at hand to give a formal definition of the common three validity models. In addition, we describe which application in practice requires which validity model, that is we give a mapping of use cases to validity models. We also analyse which standard implements which model and show how to enforce each model in practice.