The issue of choosing the right priming solution for adult cardiopulmonary bypass patients has been studied and argued for at least three decades. However, there is still no general consensus with regard to making the right choice. Basically, priming solutions can be classified into two categories, i.e. crystalloids and colloids. The former consists of dextrose, balanced crystalloid fluids, and mannitol, and the latter consists of albumin, dextrans, gelatins, and hydroxyethyl starch. In general, crystalloids are simple volume expanding solutions that mimic the normal plasma electrolyte concentrations. They can be used as clear priming solutions resulting in effective hemodilution but they lack oncotic activity. On the contrary, colloids have the advantage in maintaining the colloid oncotic pressure and reducing tissue oedema. However, colloids have been associated with increased incidence of anaphylactoid reactions and clinical coagulopathy. In this paper, we will describe the basic characteristics, the clinical efficiency and the safety of different types of priming fluids and make an overview on how to select the ideal priming solution for cardiopulmonary bypass in adults.