l-Canavanine (CAN) is a non-protein amino acid (NPAA) possessing toxic properties in both animal and plant systems. Upon treatment, this arginine structural analogue is typically incorporated into proteins by arginyl-tRNA synthetase, leading to rapid functional disruption of such “canavanyl proteins”. CAN is produced in many legumes including jack bean and lucerne (alfalfa) and is accumulated mainly in seeds and their newly germinating sprouts. It has been described as a potent allelochemical and its toxicity has been associated with autoimmunological diseases in humans or animals feeding on plants containing this NPAA. Application of CAN even at low concentration resulted in an inhibition of plant growth. When CAN was used as an anticancer agent, its mode of action appears to be associated with the synthesis of non-functional proteins in sensitive organisms, a similar mode of action to that of other simple NPAAs as meta-tyrosine. CAN toxicity in plants is also likely associated with the formation of non-functional proteins and its application has been shown to cause disruption of polyamine metabolism and formation of reactive nitrogen species including nitric oxide (NO). In higher plants, CAN has recently been used as a tool to study the regulation or modulation of polyamine–NO cross-talk. Comparing to other related NPAAs that impact cellular function in living plant and animal systems CAN seems to have the highest toxic properties. The aim of this review is to describe CAN specific activity and mode of action especially focused on higher plant systems.