Modern thermodynamics is a part of thermodynamics for complex systems, or more exactly, modern thermodynamics is a part of thermodynamics for researches on coupling systems, in which both nonspontaneous and spontaneous processes exist simultaneously. Because of the complicities of the simultaneous existence of nonspontaneous and spontaneous processes, modern thermodynamics had been blocked at its developing stage nearly through the whole of the 20th century. The term “modern thermodynamics” was not generally accepted nearly through the whole of the 20th century, or only meant the status of thermodynamics at that time. The interaction between simultaneous nonspontaneous and spontaneous processes in the same system had been called “compensation” by Clausius in 1865, but is now usually called thermodynamic coupling, which is the core of modern thermodynamics.