The reactions reviewed herein are those showing an unusual outcome when performed in water or in an aqueous medium. Notably, transformations are included for which the hydrophobic effect might be invoked as the activation factor, as well as cases where water leads to valuable changes compared to traditional solvents, except when they are clearly due to the polarity as for example in nucleophilic substitution reactions and except when the effect comes from the addition of a few equivalents of water. 1. Introduction 2. Physicochemical Background 2.1 Structure of Water 2.2 Aqueous Solutions of Simple Molecules 2.3 Hydrophobic Effects 2.4 Reactivity in Water 3. Pericyclic Reactions 3.1 Diels-Alder Reactions 3.2 Hetero Diels-Alder Reactions 3.3 Dipolar Cycloadditions 3.4 Claisen Rearrangements 4. Nucleophilic Additions 4.1 Aldolisation (Mukaiyama Reaction) 4.2 Michael-Type Reactions 4.3 Other Nucleophilic Additions 5. Organometallic Chemistry 5.1 Barbier-Type Reactions 5.2 Transition Metal Complex Catalyzed Reactions 5.3 Other Metal-Catalyzed Reactions 6. Oxidations 7. Reductions 8. Photochemistry 9. Conclusion