To overcome the problems of system theory and network theory over real field; this book uses matrices over the field F(z) of rational functions in multi-parameters describing coefficient matrices of systems and networks and makes systems and network description over F(z) and researches their structural properties: reducible condition of a class of matrices over F(z) and their characteristic polynomial separability; reducibility; controllability; observability and structural conditions of networks over F(z); and so on. This book involves three subjects: systems; networks and matrices over F(z); which is an achievement of interdisciplinary research.