Thermodynamics is a science of Energy and Entropy. It is a branch of physics that studies material properties and processes with regard to relationships between all forms of energies that ultimately dissipate into heat and generate entropy. It has been formally established rather recently, in the second half of the nineteenth century after pragmatic research in prior centuries, particularly propelled by the development of heat engines and conversion of heat to work. Sadi Carnot’s 1824 treatise, “Reflections on the Motive Power of Fire and on Machines Fitted to Develop that Power,” opened the way to further developments and generalization of thermodynamic reversibility and energy process-equivalency, the definition of absolute thermodynamic temperature (Kelvin 1848), and a new thermodynamic material property entropy (Clausius 1865), as well as the Gibbs free energy (Gibbs 1875), one of the most important thermodynamic functions for the characterization of electro-chemical systems and their equilibriums, thus resulting in the formulation of the universal and far-reaching Second Law of Thermodynamics. Even the First Law of energy conservation was not formulated until 1847 by Helmholtz, after its postulation by Mayer in 1842, and famous Jules’ 1843 experiments, establishing equivalency of work and heat, long after the 1798 Count Rumford’s (Benjamin Thompson) prominent cannon-boring experiments to demonstrate the conversion of work into heat.This section focuses on original reasoning and new research results in fundamentals and applications in thermodynamics. Original manuscripts in different areas of thermodynamics, including critical up-to-date reviews are solicited. We welcome submissions addressing novel issues, as well as those on more specific topics. It is hoped that the thermodynamics section will inspire and motivate scientists and practitioners to revisit important and critical issues related to the Laws of Thermodynamics as the most fundamental laws of nature.https://www.mdpi.com/journal/entropy/sections/thermodynamics