Plasmonics – the field of investigation of light-matter interaction on the nanometer scale, is a branch of optical engineering and nanotechnology. One category of materials that is known to enable the field of plasmonics is metal. In fact, metal nanoparticles have been consistently used to enhance optical properties of various active objects around their near fields. I will present an intuitive model with a clear physical picture to explain the origin of the optical field enhancement by metal nanoparticles and associated coupled nano-structures. Taking into account of metal ohmic loss, I shall establish limitations of such field enhancement, and finally, I will discuss the impact of Landau damping that ultimately limits the increase of field enhancement accompanied by the narrowing of gaps between plasmonic nanostructures and explain why the much anticipated giant enhancement does not always materialize in practice.