Internet of Things (IoT) is slowly but steadily becoming part of different aspects of our lives, with applications ranging from smart homes, to wearable devices, to healthcare, etc. This wide spectrum of applications results in shared data containing large amount of users' private information. The security of such information becomes a paramount concern. The IoT security requirements include data confidentiality, data integrity, authentication, access control, privacy, etc. In particular, authentication of IoT devices has a particular importance given the variety of attacks which might result from its breach [1]. This paper provides a survey of the different authentication schemes proposed in the literature. Through a multi-criteria classification, it compares and analyzes the existing authentication protocols, showing their advantages and disadvantages.