Several experimental and non experimental studies have shown that mammography every 2-3 years can reduce mortality for breast cancer in women over 50 of about the 30%. On this basis, in the last few years several programmes, population-based, have been organised at Regional and National level in Europe. In this paper the main process indicators useful to assess early the impact of a screening programme are discussed. The proposal of a system for the programme evaluation is mainly based on the experience of the Two County Study, in Sweden, the most important randomised clinical trial in this field.