This paper provides efficient design verification methodologies based on concurrent simulation technique which is implemented in the interactive simulation automation system (ISAS). Concurrent simulation is not a new technique, however, the ISAS exploits many parallelisms during the simulation procedure and applies it to three different types of simulation: concurrent (C_sim), distributed concurrent (Dc_sim), and concurrent-concurrent (CON^2FERS). ISAS automates the simulation procedure with a graphic user interface and handles not only fault models, but also design error models. It generates fault and design error coverage metrics for the user. This paper provides a description of three types of concurrent simulators with performance results.