These lectures discuss the basic processes that give rise to signals in particle detectors. The first lecture will summarize the main points from the 2019 lecture series on this topic (https://indico.cern.ch/event/843083/) and will discuss further extensions of the Ramo-Shockly theorem that are valid for the full extent of Maxwellâ€™s equations without limitation to weak conductivity and can be e.g. used for calculation of antenna signals from cosmic rays showers in the atmosphere or in ice. The second lecture discusses the theory of electron-hole avalanches leading to a detailed description of Low Gain Avalanche Diodes (LGADs), Silicon Photomultipliers (SiPMs) and Single Photon Avalanche Detectors (SPADs) that are nowadays widely applied in particle physics experiments. The third lecture will discuss the different sources of noise arising in detectors and will describe the mathematical tools for quantifying noise. The fourth lecture will then discuss signal processing and the theory of optimum filtering. The fifth lecture is foreseen for questions and specific topics that arise during the lectures.