This report is a summary of the present level of knowledge with regard to aggregates in concrete. It focuses on the composition of aggregates and the mineral and petrographical variations which affect the quality and properties of the aggregates. _x000D_ _x000D_ The report starts with a brief description of how aggregates reacts in fresh, new and hardened concrete. This information provides the basis for understanding why it is important to require a certain quality from aggregates in concrete. After that there is a chapter about the different types of rocks and their composition. The description is a brief summary of the geological know-how required to understand the variations of aggregates and the underlying reasons for many of the positive and negative properties of concrete. _x000D_ _x000D_ The second half of the report, from chapter 5 onwards, mainly deals with how to examine and describe aggregates. The description is based on the standards which are being drawn up in Europe, the so-called CEN-standards. As soon as they are ready, they will be used as Swedish standards. The regulations covering aggregates in concrete, which are based on international experience, are more comprehensive and slightly more detailed than those we have been used to in Sweden. _x000D_ _x000D_ Finally, the report contains a list of all the test methods which can be used to identify components in aggregates which can be deleterious to concrete.