Few systems in nature are entirely in equilibrium. Out of equilibrium, there are heat currents, and different degrees of freedom or parts of studied systems may be described by entirely different temperatures if the concept of temperature is at all well defined. In this project we will study the emergence of the subsystem temperatures in different types of small electronic systems, and the physical phenomena associated with those temperatures. Our emphasis is on the mesoscopic effects, residing between the microscopic world of individual atoms and electrons, and the macroscopic everyday world. In particular, we will research thermometry methods, different types of relaxation, magnitudes of fluctuations and effects at high frequencies. We will explore these effects in a wide variety of systems: normal metals and superconductors, carbon nanostructures, nanoelectromechanical and spintronic systems. Besides contributing to the understanding of the fundamental properties of electronic systems, our studies are directly relevant for the development of thermal sensors and electron refrigerators. The improved understanding of the thermal phenomena will also benefit the study of almost any type of a nonlinear phenomenon in electronics, for example the research of solid-state realizations of quantum computing or the race towards quantum limited mass and force detection.