This book lays the theoretical foundations and emphasizes the close connection between theory and experiment to optimize models and real-life procedures for the various stages of polymer composite development. As such, it covers quantum-mechanical approaches to understand the chemical processes on an atomistic level, molecular mechanics simulations to predict the filler surface dynamics, finite element methods to investigate the macro-mechanical behavior, and thermodynamic models to assess the temperature stability. The whole is rounded off by a look at multiscale models that can simulate properties at various length and time scales in one go -- and with predictive accuracy.