Mechanical behaviors of materials are investigated by various techniques on different scales. Although microstructure parameters such as grains and dislocation structure show dominant effects on the structural behavior during plastic deformation, it is also influenced by the size of the material and the deformation range. Nanoindentation methods are used to investigate mechanical behaviors of materials, especially, size effects during the plastic deformation and the associated dislocation structures. Also, macroscale mechanical properties like Young’s modulus and hardness can be determined with higher accuracy using nanoindentation methods. Application of different nanoindentation methods results in a comprehensive understanding of the deformation and dislocation mechanisms. This study focuses on the physical mechanism behind size effects for various materials by combining experimental and numerical results.