This book presents basic information about DNA, along with comprehensive theoretical introduction to DNA. It discusses recent developments in divalent-metal-ion inserted M-DNA complex, which gives rise to the possibility of DNA application to electronic functionality. Further, the book describes three examples of applications: optical and electrical materials, electronic devices such as bioTFT memory and color-tunable light-emitting diodes, and biofuel cell application with use of proton conduction in DNA.