Plants produce a wide variety of phytochemicals during biochemical reactions. Plant secondary metabolites (PSMs) are specialised metabolites or phytochemicals that are produced by plants in response to biotic and abiotic stresses or as a by-product of metabolism. PSMs are needed by the cells of the plant to interact with its environment and are produced in very small concentrations. They have various therapeutic effects and are used as medication in various conditions. However, the difference between therapeutic and toxic levels of these compounds is so low that when administered in excess, they can cause adverse conditions. These toxic PSMs are also known as phytotoxins. Phytotoxins are highly versatile in their mode of toxicity, chemical composition, and structure. The major groups of phytotoxins that have been categorised are alkaloids, terpenoids, glycosides, non-protein amino acids, glucosinolates, cardenolides, tannins, phenolics, flavonoids, and phytoecdysteroids. These phytotoxins have several toxic effects like allergenic, pesticidal, hallucinogenic, and allelochemical and may even cause fatalities. The effects of these phytotoxins in humans range from the disturbance in the metabolic pathways taking place in various organs to adverse conditions like cytotoxicity, neurotoxicity, and carcinogenicity. This chapter discusses in detail the various categories of phytotoxic compounds from land plants and their major biological activities.