This chapter explains the modern concepts of the nature of surface properties and phenomena and describes the methods of their investigation. Surface properties and surface phenomena have a significant influence on many aspects of welding and surfacing processes. One of the main properties of liquid and solid bodies is the availability of free surface energy, which is manifested in a well-known tendency of the liquid to take up a shape with minimum surface. The transition of particles will disturb the electric neutrality of both the phases and since in the conductors excess charges are driven to the surface, the electric double layer will form and the associated potential jump will occur on the metal–slag interphase. Methods to determine the surface tension of liquids are divided into two groups: dynamic and static. Static investigation methods became the most widely used, due to a greater accuracy of the experiment.