This chapter presents an overview on sensors based on polymer optical fibers (POF). It provides an overview on the principles for solid POF sensors and exemplary applications. While many sensors rely on glass-optical fibers, which are mostly single mode and thus relatively delicate in terms of handling and operation, multimode fibers, especially robust, large-core POF, are a cost-effective alternative. Most surface treatments on the interior surfaces of the holes in microstructured fibers have required the fluids to pass the full length of the fiber. Polymers have some advantageous mechanical properties that make microstructured polymer optical fibres better for mechanical sensing in some situations. The fluids have to be brought near the optical core of the fiber, which also provides some challenges. Another sensing approach uses reflections either outside or within the fiber. Attenuation is a good effect for sensor applications since power meters can relatively easily measure it.