Recently four legged robots are becoming quite robust and, therefore, can have many promising applications such as industrial surveillance. Two legged robots also have great progress toward more robust walking. However, two legged robots have an inherent stability problem due to the fact that it has only two feet. In this talk, I would like to explain the unique characteristics of two legged robots in terms of hardware and control. Two legged robots require the feet of a finite size to stand still, but these feet make the dynamic characteristics of the robot difficult to control. On the other hand, the feet are always making contacts while walking. It is challenging to deal with this continuous contact in terms of sensing and control. Among many possible approaches, some of the methods that our research group is pursuing will be also introduced in these aspects.