Current CPUs have a Trusted Execution Environment (TEE) mechanism to run a critical process in isolatation from the operating system. Famous TEEs are Intel SGX, AMD SEV, and Arm TrustZone. In addition, the open architecture “RISC-V” has some proposals for TEE implementation. Unfortunately, TEE functions depend on CPU implementation. The common function of a TEE is isolated execution only, which requires supporting technologies for secure processing. In this paper, the details of each TEE implementation as well as its security-supporting technologies, i.e., Root of Trust for critical information and Remote Attestation for verifying CPU and code integrity, are discussed. The software build environment, vulnerability, and standardization activities are also introduced.