This paper suggests an approach to the development of software testing and debugging automation tools based on precise program behavior models. The program behavior model is defined as a set of events (event trace) with two basic binary relations over events -- precedence and inclusion, and represents the temporal relationship between actions. A language for the computations over event traces is developed that provides a basis for assertion checking, debugging queries, execution profiles, and performance measurements. The approach is nondestructive, since assertion texts are separated from the target program source code and can be maintained independently. Assertions can capture the dynamic properties of a particular target program and can formalize the general knowledge of typical bugs and debugging strategies. An event grammar provides a sound basis for assertion language implementation via target program automatic instrumentation. An implementation architecture and preliminary experiments with a prototype assertion checker for the C programming language are discussed.