Secondary metabolites of plants are a diverse class of compounds that play central roles in plant defense, communication, and adaptation to ecological stresses. This section covers the outline of plant bioactive compounds, covering their classification, biosynthetic pathways, ecological functions, and significance in human health and industry. Secondary metabolites consist of various chemical classes, such as alkaloids, terpenoids, phenolics, and glucosinolates, each characterized by distinct structures and biological properties. Their biosynthesis is regulated by complex metabolic pathways involving enzymes and precursor molecules, often influenced by environmental factors and developmental stages. In ecological terms, secondary metabolites play a pivotal role in safeguarding plants against herbivores, pathogens, and environmental pressures, while also regulating interactions with other organisms. Moreover, many secondary metabolites possess pharmacological properties beneficial to human health, serving as sources of therapeutic drugs, nutraceuticals, and dietary supplements. Furthermore, these compounds find utility across diverse industrial sectors, spanning agriculture, cosmetics, and food manufacturing. Understanding the biosynthesis, ecological roles, and industrial potential of plant secondary metabolites is essential for advancing research in plant biology, ecology, and biotechnology, as well as for harnessing the potential of natural products for human welfare and sustainable development.