“Nothing is in the intellect that was not first in the senses.”—A famous saying which is attributed to John Locke applies to the knowledge of robots as well. Most information processed by robots is perceived by sensors. All sensor measurements are affected by noise and may be faulty. A crucial challenge is therefore how to perceive the environment and how to handle sensor data. This chapter discusses important sensors for mobile and industrial robots: motion sensors, cameras, structured-light and time-of-flight sensors, laser range finders, and stereo-cameras. Associated algorithms are needed to process such sensor data. Most of the algorithms use a probabilistic model and/or optimization with a specific target function.