This document provides an overview of nano-sensors. It begins with defining a sensor as a transducer that converts a measurement into a signal. Nano-sensors are then defined as being tiny sensors in the size of a few nanometers, around 10-100 nanometers, that can detect nanomaterials or molecules of similar size or smaller. The document goes on to describe four main types of nano-sensors: optical nano-sensors including proximity and ambient light sensors, biological nano-sensors, chemical nano-sensors, and physical nano-sensors. Examples are given for each type.