Quantitative modeling of biopathway dynamics is crucial to the system-level understanding of cellular functions and behavior. Currently, a common method of representing bio-pathways is through a system of ordinary differential equations (ODEs). However, calibrating and analyzing large ODE-based pathway models often requires a large number of numerical simulations. To address this issue, this book presents a probabilistic approximation approach. by which the analysis of ODE-based models can be considerably simplified. The applicability of these techniques has been by studying two existing pathways and a "live" pathway called the complement system in collaboration with biologists. Apart from improved performance, the crucial insights gained from the study of complement system could contribute to the development of immunomodulation therapies. This book should be useful to researches in Computational Systems Biology field, or anyone else who may be considering utilizing ODEs for their modeling efforts.