Several phytochemicals, belonging to different classes of plant constituents, have been identified as causative agents for evoking toxicity/health threats to humans. The range of toxic effects includes mild symptoms of damage to vital organs, carcinogenicity, mutagenicity, and occasionally the toxicity leads to death. In the following chapter, three major classes of constituents, widely distributed in plants, possessing potent toxicity, and found to be major sources of inflicting toxicity in humans, will be discussed. These are pyrrolizidine alkaloids, steroidal glycoalkaloids, and furanocoumarins. For every class, the plant sources for the toxic components, the relevant chemical properties, the effect on the human body, and in a few cases the mechanism of toxicity will be mentioned. The factors affecting accumulation or the level of the toxic components in the plant/food will also be discussed. © 2021 Elsevier Inc.