Clicker training is a popular technique used in companion animal training. It employs a handheld signalling device called a clicker, which emits an audible “click” noise when pressed. Trainers press the clicker when an animal performs a desired behaviour, usually following the click with presentation of a food reward. The clicker is purported to facilitate learning, but scientific evidence to support this claim is limited. Of five studies comparing a clicker-type signal + food group with a food-only control group, only one found that animals in the signal + food group learned faster. Further investigation is therefore required to better understand the circumstances under which clickers might help or hinder learning. To inform future studies, it is important to consider mechanisms by which the clicker may function. In this paper three proposed mechanisms are presented, which we term the Reinforcing Hypothesis, Marking Hypothesis, and Bridging Hypothesis. To begin understanding which (if any) of these three mechanisms is the means by which clickers may operate, we evaluate relevant laboratory animal studies. Based on available behavioural and neuropsychological evidence, it is concluded that clickers and other clicker-like stimuli likely function as conditioned reinforcers, but may also have marking and bridging properties. Ways to investigate how this translates to clicker use in applied settings are identified.