Tactile sensing has been recognised as an important sensing modality in robotic object recognition and manipulation tasks. There are two main robotic tactile sensors: those flexible and lower resolutions intended for robot skin applications, and those intended for robot hand and finger applications that are typically higher resolution. The tactile sensors intended for use as robot body skin are typically lower resolution sensors and in many cases can be simple sensors that detect when the skin has been touched. Depending on the intended sophistication of the application, these can be simple binary on-off sensors or more complex sensors that detect normal and tangential forces acting on the skin. To fulfil all robotic tactile sensory needs with a single type of sensor is close to impossible. Therefore most transduction principles have been used in the tactile sensor development including capacitive, resistive/conductive, piezoelectric, pyroelectric, piezoresistive, optic, ultrasonic, magnetic, magneto-electric, mechanical (on/off) and quantum tunnelling. This paper briefly presents the working principles of sensors in these categories.