This book presents the multi-component first-principles methods which can take account of the quantum effect of light particles such as proton and positron, as well as electron. In particular, the authors introduce their multi-component molecular orbital (MC_MO) methods and multi-component "hybrid type" density functional theory (MC_DFT). Using these multi-component procedure, the authors can analyse many chemical phenomena, H/D isotope effect, positronic systems, and so on. The authors show some examples of MC_MO and MC_DFT works.