Devices for electrochemical energy conversion and storage exist at different levels of development, from the early stages of R&D to mature and deployed technologies. Thanks to the very significant progresses achieved in the field of computational science over the past few decades, multiscale modeling and numerical simulation are emerging as powerful tools for in silico studies of mechanisms and processes in these devices. These innovative approaches allow linking the chemical/microstructural properties of materials and components with their macroscopic efficiency. In combination with dedicated experiments, they can potentially provide tremendous progress in designing and optimizing the next-generation electrochemical cells. This chapter provides a comprehensive overview of the theory and practical aspects of integrative multiscale modeling tools within the context of fuel cells and rechargeable batteries. Additionally, the chapter discusses technical dreams and methodological challenges that computational science is facing today in order to help developing efficient, durable, and low-cost electrochemical energy devices but also to trigger major technological breakthroughs.