Authentication is a mechanism to establish proof of identities. Common practise for authentication is to challenge user to present authentication credential. User who can present the valid credential is considered as authenticated identity. Traditional authentication systems have a static security requirement which does not consider the change of user behavior or environment. In contrast, adaptive authentication systems are risk-based authentication that consider those changes to identify high-risk and suspicious illegitimate login attempts. In this paper, we examine some of the adaptive authentication systems that have been proposed recently in the literature. Some of the issues and challenges when developing an adaptive authentication system are also discussed. We also introduce our own Unified Authentication Platform (UAP) which incorporates adaptive control. We describe two typical processes that are used in our Adaptive UAP.