To achieve high fidelity and high credibility of a microscopic traffic simulation, model validation is of the utmost importance. A systematic validation effort was performed on an advanced microscopic traffic simulation that uses a multiregime simulation (MRS) approach for its car-following and lane-changing logic. The vigorous validation procedure includes animation comparison and quantitative/statistical analysis at both macroscopic and microscopic levels, and is illustrated through case studies. For validation at the macroscopic level, the averages and other statistics of traffic variables are compared, and fundamental relationships of traffic flow parameters are studied. For validation at the microscopic level, the speed change pattern, vehicle trajectory plots, and headway distribution from simulations are compared with those of the real-world system. The importance of the real-world data sets to model validation is emphasized, and animation comparison that provides intuitive appreciation of the model validity is frequently used. The validation and testing not only show the validity of the MRS model but also illustrate a systematic approach that can be used to validate other simulation models.