This is an introduction on using quantum systems as sensors. These sensors can utilize quantum mechanical properties to enhance their performance for detecting classical, real world signals. Moreover, they allow access to an unprecedented regime where a single molecule or even a single electron can be detected, and their nanoscale dynamics can be observed. The talk will begin with a discussion of general sensing applications and a brief overview of how quantum mechanics can be utilized for sensing. Then, we will look at a few examples of quantum sensor platforms before focusing on the topic of using defects in diamond as a quantum system and quantum sensor.