Hydroxycinnamic acid compounds are an important source of antioxidants due to their ubiquitous occurrence in the plant kingdom and their characteristic activities. Due to their antioxidant activity, several researchers have attributed a probable role of these compounds in the prevention of various diseases associated with oxidative stress, such as cancer and cardiovascular diseases. Recent evidence suggests that these compounds may also act by other mechanisms in addition to the antioxidant capacity as modulating the activity of some specific enzymes and inhibit cell proliferation. This paper is a comprehensive review of the effects of hydroxycinnamic acids on cancer. The review encompasses the occurrence and bioavailability of these compounds evidences for their effects on cancer and the various mechanisms by which may exert their effects. There are several common mechanisms by which these chemicals exert their effects that could be conducive to additive, synergistic, or antagonistic interactions. These include effects on cellular differentiation, proliferation, and apoptosis; effects on proteins and enzymes that are involved in these processes at a molecular level, and other various effects through altered immune function and chemical metabolism.