In this chapter, we have introduced the fundamentals of self-sustaining wireless communication networks. We have first provided the overviews of conventional energy harvesting networks, i.e., wireless-powered transfer, wireless-powered communication network, and simultaneous wireless information and power transfer, as well as their applications in the literature. Then, we have introduced ambient backscatter communications in terms of architecture, design, advantages, and limitations. Finally, we have discussed potential applications and implementation of ambient backscatter communication system networks such as smart world, biomedical, and logistics.