Surface tension plays an important role in multi-phase systems. Its effect becomes especially dominant in micro-scales because the ratio of surface (interface) to volume is relatively large. To understand such surface tension effects, it is required to understand the surface tension itself as a thermo-physical property and at the same time, its roles in fluid dynamics and heat and mass transfer. For the better understanding of surface tension effects, it is further recommended to learn some typical surface tension phenomena So in this article, the fundamental matters relevant to surface tension are firstly described, and then the action of surface tension in fluid dynamics as well as thermal-fluid systems is discussed by addressing similarity laws and dimensionless quantities with special attention to scaling effects. Surface tension problems, besides the two sides of thermo-physical property and the effects in thermo-fluid systems, include another important related problem, that is, the interaction of liquid with contacting solid surfaces. The last part of this article includes three recent topics, each of which relates to each side of the surface tension problems categorized above: i.e. a new precise measurement method of surface tension, flow patterns and heat transfer characteristics of two-phase flow in micro conducts, and solid surface modifications and their applications to fluidic devices.