Since ancient time immemorial, plants have been omnipresent in therapy. Many records of ancient civilizations have chronicled the use of plants and extracts thereof for disease remediation. Their presence in modern therapy is ubiquitous. Studies investigating the role of primary and secondary metabolites in plant growth and survival led scientists to explore the role of secondary metabolites in modern therapy. As newer analytical and instrumentation techniques were developed, identification, categorization, and characterization of secondary metabolites gained importance and a battery of scientists worldwide conducted research to study their potential in therapy of many diseases. The secondary metabolites were renamed as phytoconstituents. Their relative safety in terms of reduced adverse effects, multipronged mechanisms of action, and better pharmacokinetic profiles as compared to many synthetic drugs make phytoconstituents superior therapeutic agents. The range of phytoconstituents with diverse chemical structures include flavonoids, coumarins, terpenoids, polyphenolic acids, alkaloids, glycosides, and organo-sulfur compounds, to name a few. They display a plethora of activities like anticancer, antioxidant, antimicrobial, and immunomodulatory activities. This multifunctionality is the most attractive feature of phytoconstituents. There are hundreds of phytoconstituents which have shown great promise in in vitro biochemical and cell line studies. Many have entered into therapeutic regimen either as medicines themselves or playing a supportive role to existing treatment modalities. Chronicling the entire gamut of phytoconstituents in therapy is an onerous task. This chapter covers the fundamentals of secondary metabolites in modern therapy, the history, classification, and some phytoconstituents which have been studied for or are being used for treatment of cancer, neurodegenerative disorders, and disease involving immune system dysfunction.