Cannabinoids, the active components of the hemp plant Cannabis sativa, along with their endogenous counterparts and synthetic derivatives, have elicited anti-cancer effects in many different in vitro and in vivo models of cancer. While the various cannabinoids have been examined in a variety of cancer models, recent studies have focused on the role of cannabinoid receptor agonists (both CB(1) and CB(2)) in the treatment of estrogen receptor-negative breast cancer. This review will summarize the anti-cancer properties of the cannabinoids, discuss their potential mechanisms of action, as well as explore controversies surrounding the results.