Book excerpt: This book is devoted to the study of the semantics and assuring the efficient execution of database query languages, where the database contains semistructured and time-varying information. The primary focus of the work presented is the definition of an effective graph-based approach to the formalization of query languages for semistructured and temporal information; as a result, query execution can be reduced to searching the database for subgraphs that are similar to the given query graph and thus can be supported through bisimualtion. By integrating and refining graph-based methods and bisimulation, the author develops a powerful and flexible paradigm. The second research challenge solved is that of efficient implementation by modeling graphical queries as formulas in modal logic and interpreting database instance graphs as Kripke transition systems.