**Abstract**

Natural products are substances that are confined from living organisms, they are in the form of primary or secondary metabolites. Secondary metabolites are compounds with varied chemical structures, produced by some plants and strains of microbial species. Unlike primary metabolites (nucleotides, amino acids, carbohydrates, and lipids) that are essential for growth, secondary metabolites are not. Secondary metabolites are produced or synthesized during the stationary stage. In this chapter, we will discuss secondary metabolites from natural products synthesized mainly by plants, fungi, and bacteria. Plants synthesize a large diversity of secondary metabolites; plant secondary metabolites are split into four groups namely alkaloids, phenolic compounds, terpenoids, and glucosinolates. Several classes of fungal and bacterial secondary metabolites, their sources, and pharmacological uses associated with the secondary metabolites are also discussed. Therefore, several classes of secondary metabolites are responsible for the biological and pharmacological activities of plants and herbal medicines.

**Keywords:** secondary metabolites, natural products, alkaloids, phenolic compounds, terpenes
