The STIFF-FLOP robotic arm is a cylindrical structure made from soft silicone rubber materials encasing pneumatic actuation chambers. Its material properties and structure allows for the shape, and therefore the pose of the arm, to be dictated by the actuation system as well as by the surfaces with which it is interacting. Although this softness and flexibility makes it inherently safe for many medical and industrial applications, such as keyhole surgery, it comes at the expense of complicating sensing and position control. This chapter presents the main challenges for the development of a pose sensor for soft robotic arms and the STIFF-FLOP approach to tackle them.