This review provides a comprehensive analysis of biostimulants in plant science, focusing on their biological basis, classification, and regulatory framework. Biostimulants are defined as formulated products of biological origin that improve plant productivity through the novel or emergent properties of their constituents, rather than solely through known plant nutrients or growth regulators. The review highlights the challenges in defining biostimulants due to their diverse sources and complex compositions, and emphasizes the need for a science-based approach to their development and regulation. It discusses the various methods used to study biostimulants, including chemical and non-chemical characterization, plant growth studies, and -omics technologies. The review also addresses the classification of biostimulants based on their origin, mode of action, and functional properties. It notes the importance of understanding the mechanisms of action of biostimulants, even if the exact mode of action is not fully understood, and highlights the need for regulatory frameworks that ensure the safety and efficacy of these products. The review also discusses the diverse sources of raw materials used in biostimulant production, including microorganisms, algae, higher plants, and animal-derived materials. It emphasizes the complexity of biostimulant compositions and the challenges in determining their modes of action. The review concludes that a science-based approach is essential for the development and regulation of biostimulants, and that further research is needed to fully understand their biological functions and potential applications in agriculture.This review provides a comprehensive analysis of biostimulants in plant science, focusing on their biological basis, classification, and regulatory framework. Biostimulants are defined as formulated products of biological origin that improve plant productivity through the novel or emergent properties of their constituents, rather than solely through known plant nutrients or growth regulators. The review highlights the challenges in defining biostimulants due to their diverse sources and complex compositions, and emphasizes the need for a science-based approach to their development and regulation. It discusses the various methods used to study biostimulants, including chemical and non-chemical characterization, plant growth studies, and -omics technologies. The review also addresses the classification of biostimulants based on their origin, mode of action, and functional properties. It notes the importance of understanding the mechanisms of action of biostimulants, even if the exact mode of action is not fully understood, and highlights the need for regulatory frameworks that ensure the safety and efficacy of these products. The review also discusses the diverse sources of raw materials used in biostimulant production, including microorganisms, algae, higher plants, and animal-derived materials. It emphasizes the complexity of biostimulant compositions and the challenges in determining their modes of action. The review concludes that a science-based approach is essential for the development and regulation of biostimulants, and that further research is needed to fully understand their biological functions and potential applications in agriculture.