The purpose of this book is to show the main techniques and strategies needed to design and model simple and complex photonic devices, which can be used for telecommunications, signal processing or sensing applications. In fact, the modelling approaches and techniques of photonic devices are not yet well standardised. In many fields, in particular Silicon Photonics or Polymer Photonics, a few experimental devices have been presented in literature, but their fundamental modelling and design aspects are still completely or partially ignored. The high-quality research and review chapters are written by the members of the Photonics Research Group.