One of the interesting properties of metal nanoparticles is their optical properties which varies with nanoparticles¿ shape, size and material and also with refractive index or dielectric constant of surrounding medium of nanoparticle. Nanostructured metals show very complex and interesting optical properties because of the excitation of surface plasmons in them. The excitation of surface plasmons in metallic nanoparticles induces optical properties hardly achievable in other optical materials, yielding a wide range of applications in many fields. Surface plasmon resonance is the most outstanding optical property of metallic nanostructures. This book is arranged in three chapters.The first chapter ¿An overview of nanotechnology and plasmonic¿ is dedicated to the background of the research and the review of the resources in this field. In the second chapter ¿Fundamentals of Plasmonics- Theoretical background¿, the concepts, solutions and mathematical tools used in this field are presented. And in the third chapter ¿Plasmon in big nanoparticles, Mie theory¿, we will discuss the research conducted in this field, and present the obtained results at the end of each sub-section.