Water, a fundamental component of life, is a preferred solvent for natural chemical synthesis. However, most organic reactions in laboratories and industry are hindered by water or oxygen. To mimic nature, chemists use highly reactive nucleophilic and electrophilic reagents, often requiring protection of functional groups. Despite this, aqueous reactions have been studied for decades, with early examples like the synthesis of indigo in 1882. Water's unique properties, such as high dielectric constant and solubility, enhance reaction rates and selectivity. However, its use in organic synthesis is limited due to solubility issues, prompting the use of co-solvents. The "on water" approach, where reactions occur in or on water without organic solvents, has gained traction. This method offers advantages like reduced cost, simplicity, and ease of product isolation. Reactions on water can be homogeneous or heterogeneous, with varying water amounts. The term "on water" is often used interchangeably with "in water" or "with water." Recent studies show that reactions on water can significantly accelerate rates and improve selectivity, especially for water-insoluble substrates. Examples include Diels-Alder reactions, 1,3-dipolar cycloadditions, and nucleophilic ring openings. Water's role in these reactions is crucial, often enhancing rates through hydrophobic effects and improving selectivity. Despite ecological benefits, the environmental impact of a process depends on factors like atom economy and waste management. The review highlights various reactions performed on water, including those catalyzed by transition metals and metal-free processes, demonstrating the potential for process intensification and green chemistry.Water, a fundamental component of life, is a preferred solvent for natural chemical synthesis. However, most organic reactions in laboratories and industry are hindered by water or oxygen. To mimic nature, chemists use highly reactive nucleophilic and electrophilic reagents, often requiring protection of functional groups. Despite this, aqueous reactions have been studied for decades, with early examples like the synthesis of indigo in 1882. Water's unique properties, such as high dielectric constant and solubility, enhance reaction rates and selectivity. However, its use in organic synthesis is limited due to solubility issues, prompting the use of co-solvents. The "on water" approach, where reactions occur in or on water without organic solvents, has gained traction. This method offers advantages like reduced cost, simplicity, and ease of product isolation. Reactions on water can be homogeneous or heterogeneous, with varying water amounts. The term "on water" is often used interchangeably with "in water" or "with water." Recent studies show that reactions on water can significantly accelerate rates and improve selectivity, especially for water-insoluble substrates. Examples include Diels-Alder reactions, 1,3-dipolar cycloadditions, and nucleophilic ring openings. Water's role in these reactions is crucial, often enhancing rates through hydrophobic effects and improving selectivity. Despite ecological benefits, the environmental impact of a process depends on factors like atom economy and waste management. The review highlights various reactions performed on water, including those catalyzed by transition metals and metal-free processes, demonstrating the potential for process intensification and green chemistry.