Bridging the gap between traditional books on quantum and statistical physics, this series is an ideal introductory course for students who are looking for an alternative approach to the traditional academic treatment.This pedagogical approach relies heavily on scientific or technological applications from a wide range of fields. For every new concept introduced, an application is given to connect the theoretical results to a real-life situation. Each volume features in-text exercises and detailed solutions, with easy-to-understand applications.This third volume covers several basic and more advanced subjects about transitions in quantum and statistical physics. The first part describes how the quantum statistics of fermions and bosons differ but also under what condition they both can merge into the classical-particle-statistics framework seen in Volume 2. It is also the opportunity to describe the fundamentals of conductors, semi-conductors, superconductors, superfluids and Bose-Einstein condensates. The second part introduces time-dependent transitions between quantum states. The time evolution of a simple two-level model gives the minimum background in order to provide more details on the laser's principle and its numerous uses. The time-dependent perturbation theory is then presented. Typical applications such as standard approaches to the scattering of massive particles (mostly neutrons, which are an archetype) are discussed. A semi-classical treatment of the electromagnetic field-matter interaction is also described. Illustrations are taken from a variety of processes such as scattering from phonons, charge distribution or spin densities. The third and last part of the book gives a brief overview of quantum electrodynamics basics with applications to photon absorption or emission spectroscopies and a range of scattering regimes. There follows a short introduction to the role of multi-photon processes in quantum-entanglement based experiments.