Reliable and accessible authentication techniques are required to maintain privacy and security. This is paramount as technology plays an increasing role in our lives. In this paper, we examine the previous work on accessible authentication techniques for blind/low vision people, deaf/hard-of-hearing people, people with cognitive impairments, and people with motor impairments. We seek to identify gaps in the current research to advocate where future efforts are needed to create authentication techniques that will work for everyone. We found a lot of variability in prior work investigating the accessibility of authentication techniques, including shortfalls and gaps in the literature. We make recommendations on the directions future research should take.