This book presents a constitutive modeling approach to include the effect of elastic, plastic and creep deformations in materials leading to softening and failure. It is based on Desai's Disturbed State Concept (DSC). Development and numerical implementation of the rate dependent modeling is proposed and detailed. The model is validated for Lead free solders in electronic packaging and Glacial tills. The book presents basic theory of constitutive modeling, plasticity, creep and representation of damage mechanisms such as fatigue and fracture.