A short overview of main characteristics of the impactor population from which major terrestrial impacts originated is given. This population includes the objects that may hit the Earth in the future (potentially hazardous asteroids, PHAs). An impact frequency (a way of measuring the probability of a given collision) versus impact energy (an index of the impact consequences) relationship is described on the basis of this analysis. The current state of actions started planetwide by the most developed countries to face the threat represented by an asteroid collision with the Earth is summarized. The `Torino scale', which assesses the risks connected with a discovery of a PHA in a simple and clear way is finally described. To cite this article: M. Fulchignoni, M.A. Barucci, C. R. Physique 6 (2005).