Abstract It is impossible to make calculations, which are generally valid. There are too many assumptions which must be established before an individual case can be calculated, e.g. concerning volume treated, operating period and membrane life. However, one can think of an 'average' situation looked at from such a systems level that five components can be distinguished, i.e. 1. Pretreatment 2. RO Membranes 3. Post-treatment 4. Support systems 5. Acceptance test. From this point of view one can easily find crucial factors such as membrane, operating/cleaning labour and depreciation costs. Different results are reported in the literature. It looks like some industries are more suitable than others from an economic point of view, e.g. wood-pulp, dairy and foodstuff. Generally there are questions which are seldom asked. Such are the role of opportunity costs, start-up problems and costs from help of experts. There are several advantages from using membrane technology for concentration, e.g. less need of energy, smaller installations and environmental advantages. These factors will be stressed even more in the future. Membrane technology has therefore come to stay.