热力学系统的平衡定义为热力学系统诸属性不随时间变化。通常而言，系统平衡时要比未平衡时容易研究得多。通常在研究一个热力学过程时，可以假设每个中间过程中，系统处于平衡态。这样可以大大简化对于系统的研究分析。 对于可逆过程，其每一阶段也必须是可逆的。而一个过程的可逆阶段中，系统必须时时处于平衡。而这在现实中不能达到，只能通过使过程缓慢进行来趋近，因为过程中的每一阶段在没有打破系统平衡的前提下是不能进行的。

• 秦允豪. 《普通物理学教程 热学》第三版. 高等教育出版社. 2011. ISBN 978-7-04-030090-1.
• Eu, B.C. (2002). Generalized Thermodynamics. The Thermodynamics of Irreversible Processes and Generalized Hydrodynamics, Kluwer Academic Publishers, Dordrecht, ISBN 1-4020-0788-4.
• Bailyn, M. (1994). A Survey of Thermodynamics, American Institute of Physics Press, New York, ISBN 0-88318-797-3, p. 20.
• Tisza, L. (1966). Generalized Thermodynamics, M.I.T Press, Cambridge MA, p. 119.
• Bailyn, M. (1994). A Survey of Thermodynamics, American Institute of Physics Press, New York, ISBN 0-88318-797-3, p. 22.
• Perrot, Pierre. A to Z of Thermodynamics. Oxford University Press. 1998. ISBN 0-19-856552-6.
• The Blue Book on Environment by Richard Dawkins
• Shavit, A., Gutfinger, C. (1995). Thermodynamics. From Concepts to Applications, Prentice Hall, London, ISBN 0-13-288267-1, Chapter 6.
• Adkins, C.J. (1968/1983). Equilibrium Thermodynamics, third edition, Cambridge University Press, Cambridge UK, ISBN 0-521-25445-0, pp. 46–47.
• Bailyn, M. (1994). A Survey of Thermodynamics, American Institute of Physics Press, New York, ISBN 0-88318-797-3, pp. 19–23.
• I.M.Kolesnikov; V.A.Vinokurov; S.I.Kolesnikov. Thermodynamics of Spontaneous and Non-Spontaneous Processes. Nova science Publishers. 2001: 136. ISBN 1-56072-904-X.
• A System and Its Surroundings. ChemWiki. University of California - Davis. [May 2012].
• Hyperphysics. The Department of Physics and Astronomy of Georgia State University. [May 2012].
• Bryan Sanctuary. Open, Closed and Isolated Systems in Physical Chemistry,. Foundations of Quantum Mechanics and Physical Chemistry. McGill University (Montreal). [May 2012].
• Material and Energy Balances for Engineers and Environmentalists (PDF). Imperial College Press. : 7 [May 2012].
• Abbott, M.M.; van Hess, H.G. Thermodynamics with Chemical Applications 2nd. McGraw Hill. 1989.
• Halliday, David; Resnick, Robert; Walker, Jearl. Fundamentals of Physics 8th. Wiley. 2008.
• Moran, Michael J.; Shapiro, Howard N. Fundamentals of Engineering Thermodynamics 6th. Wiley. 2008.