Abstract: Many multi-scale mechanical problems concerning multiple physical levels arecrucial in aeronautic and astronautic safety. Multi-scale problems aregreat challenges with respect tocurrent mechanical concepts and theories. In this paper, taking spallation in Al alloy and brittlefracture in rocks as typical examples, we discuss typical features, difficulties and potentialsolutions to multi-scale problems. It is shown that a reasonable characterization and treatment ofthe trans-scale coupling and trans-scale sensitivity are keys to the multi-scale problems.