In this chapter, we describe the fundamental features of surface tension from both thermodynamic and molecular perspectives. In the first section, we briefly introduce the fundamentals of the thermodynamics, such as the Gibbs and Helmholtz free energy, and then discuss their increments with respect to changes in surface area, which causes the surface tension. In the second section, first a brief history of the research on the surface tension is introduced, and then we provide the microscopic understanding of the surface tension, which needs basic knowledge of thermodynamics, statistical mechanics as well as continuum mechanics. By introducing the intermolecular interaction potential and temperature definition, and by showing conceptual pictures including some results obtained by molecular dynamics simulations, the authors hope that the target readers of undergraduate level students would find fascinating aspects of surface tension as the boundary of macroscopic and microscopic physics.