Washing of red cells is sometimes performed to reduce allergic reactions due to contaminating plasma proteins or to reduce the concentration of potassium accumulating in the supernatant of red cells during storage as an alternative to transfusion of fresher red cells in patients at risk of hyperkalaemia. There are a variety of methods for washing red cells, and the laboratory data suggest that variables such as age of red cell before washing, washing method and solution, storage medium and length of storage time after washing can all effect the final red cell quality. Studies suggest that washing removes 90-95% plasma, but the proportion of units below 0·05 mg/dl IgA (equivalent to IgA deficient) is variable dependent upon methods used. Although potassium levels are reduced immediately following washing, the rate of leakage following subsequent storage is method-dependent, requiring careful consideration of shelf life if potassium reduction is the goal. Other markers of red cell quality such as haemolysis are negatively impacted by washing so a reduced shelf life compared to standard red cells is appropriate, especially following irradiation. Data from animal models and clinical studies on possible additional benefits of washing, such as reduced lung or kidney injury, are mixed, ranging from some benefit to some harm, and further studies are warranted.© 2020 International Society of Blood Transfusion.