New modeling techniques are constantly evolving with rapid advancements in the fields of Environmental Science. One of the questions that environmentalists face is, “What set of performance measures are to be used in model validation?” since there is no study that provides a comprehensive review of the performance measures. This book answers that question by presenting a systematic approach to the operational evaluation of outdoor and indoor air quality models; on the basis of an extensive literature review. A summary table lists the ranked model performance measures and their acceptable ranges, to evaluate the extreme-end and mid-range outdoor and indoor air quality model predictions. There are seven different statistical indicators that could be commonly used to validate models, regardless of the target modeling phenomenon. The book also provides a practical perspective, by inclusion of five different real-world case studies. These five cases studies will help environmental professionals understand the model evaluation procedure and they can easily apply the same to validate their chosen models and collected data using available software.