All classical authentication protocols are based on pre-shared authentic information such as long-term secret keys or a public key infrastructure. However, there are many practical settings, where participants can additionally employ authentic Out-Of-Band (OOB) communication, e.g., manual message transfer. In this paper, we study the corresponding user-aided message authentication and key agreement protocols. In particular, we give a unified treatment of many previous results and outline common design principles. We also show that certain properties of user-aided protocols simplify the security analysis in complex environments compared to the standard authentication protocols.