The book illustrates on how one could develop a computer simulation model of an existing or proposed system using a modular approach. The modularization-concept assumes that the entire system is made up of a set of smaller manageable components (building blocks) that can be linked together to form a complete system model. The smaller manageable components are modeled separately and tested before being used in the entire model. The author shows how random variables are generated from data series to suit stochastic nature of the processes. In the book, it is also shown how a single simulation Excel workbook with several worksheets could be used as a user-interface where users may change data for each modelling scenario. Visual Basic for Application subroutines are introduced. The subroutines could be used to read data from the simulation workbook and update respective data in the model. The book also illustrates how model verification and validation could be done.