Smartphones are now handling personal and extremely important data and applications. This increases mobile environment threats and makes smartphones one of attackers' preferred targets. To mitigate threats, many proposals and counter-measures have been proposed. In this paper, we focus on one of them, the Trusted Execution Environment (TEE) which is a new family of architecture having its own hardware and software environment completely isolated from the mobile ones. It intends to offer better security than the one provided by mobile platforms. It proposes for example secure storage to applications running on top of the mobile Operating System. We give a complete overview of standardization activities on TEE. We first detail the software and hardware architecture of the TEE as introduced by GlobalPlatform specifications. Then, we describe the different available TEE solutions. Finally, we compare these solutions according to key performance parameters with a special interest to security aspects.