AbstractCationic starches are widely used as wet end additives in paper making. They provide many benefits, classified into 4 categories: improvement of mechanical strength; better retention of fines and fillers; faster drainage; reduction of waste water pollution.According to the accepted theory, a 2‐part electrical layer is formed at the solid liquid interface. The zeta potential (existing at the interface between the 2 layers) is one important parameter which influences the retention of most ionic wet‐end chemicals and governs many of the processes involved in making a functional paper product. Practical experiences showed us how to influence the zeta potential in order to obtain the desired benefits. The parameters depending on starch, which have been examined, are: the type of starch (origin), the level of addition and where it is used in the paper making process. Other influences come from the alum content, pH, occurance of shear forces and the presence of dissolved anionic material. Comparative laboratory tests have led to a better understanding of the rather complex interactions on which cationic activity is based. In this paper also the application of cationic starch in surface sizing compared to other starch modifications is discussed. Some important advantages, such as lower waste water pollution and better mechanical strength for the same pick‐up, are cl