The term "Surface Plasmons, SP" was first coined in the middle of the twentieth century to study the interaction of plasma oscillations with the electrons on the surface of metal foils. Surface Plasmons have a wide variety of applications such as in Terahertz spectroscopy. In the literature, Surface Plasmons are frequently related to Surface Electromagnetic Waves, SEW, which were first studied by Zenneck and independently by Sommerfeld in the early 1900's. However, Zenneck and Sommerfeld surface waves are rarely examined critically in the current literature on SP. Looking for a solid understanding for the relation between SP and SEW, it was necessary to study Sommerfeld's work thoroughly. The revisiting of Sommerfeld's work on Surface waves led to some important conclusions which are communicated in this paper.