Double-hydrophilic block copolymers are a new class of amphiphilic molecules of rapidly increasing importance with unique and fascinating properties potentially connecting materials science, pharmacy, biochemistry and polymer science. Characteristic of these polymers is their application in aqueous environments and that amphiphilicity is just induced in the presence of a substrate or by temperature and pH changes, respectively. Their chemical structure may be tuned for a wide range of applications covering as different aspects as stabilization of colloids, crystal growth modification, induced micelle formation, polyelectrolyte complexing towards novel drug carrier systems. As the potential of this novel polymer class is relatively unexplored yet, it can be expected that more applications will arise due to the possibility to adapt the chemical structure to either the desired substrate in contact with water of the stimulus for the induction of structural changes. This review describes the synthetic strategies towards these AB block copolymers, as well as their applications.