A collection of 21 papers, this special issue published in Entropy MDPI (ISSN 1099-4300) highlights the importance of the theory of nonequilibrium thermodynamics. The special issue, dedicated to Professor Signe Kjelstrup on the occasion of her 75th birthday, explores the theory's fundamental basis in entropy production. Guest edited by Prof. Dr. Dick Bedeaux and Prof. Dr. Alex Hansen of Norges teknisk-naturvitenskapelige universitet (NTNU), Norway and Prof. Dr. Fernando Bresme of Imperial College London, UK. Access the free MDPI Books reprint: https://brnw.ch/21wXIt9 #mdpi #openaccess #entropy #research #mdpibooks