The Risk in Protection Activities Against Floods. The human societies were attracted from the beginnings by the riversides; the people took the risk of living there. During the centuries many kind of complex hydraulic structures have been constructed for ensuring the safety of the human settlements. The safety – risk relation define the base of the relationship between society and water. The safety measurements are based on the highs and stability of the hydro technical defense constructions. The probability of the defense line damaging depends on safety probability but on risk probability, too. To avoid the disasters, the risks must be known, anticipated, measured and quantified. Only in this way the society can take the right decisions for reducing the effects of the disasters. The risk is express by the product of the probability of event and his consequences. The flooding risk is influenced by a lot of natural and human factors. That is the reason why are so many ways to calculate it. Each of them is based on the design flood size, on the flooding probability and on economical evaluations of the profit-expenses ratio. The maximum risk is come true by the events with medium frequency and consequences.