Book excerpt: This book will introduce the concept of molecular communications and nanonetworks. The publication addresses why nanoscale communication is needed for the sophisticated nano and biotechnology applications. The text introduces the frontier applications of the molecular communication and nanonetworks. The book examines the molecular communication types called active, passive, and gap junction molecular communications. The author presents the molecular transmitter, receiver, encoding and decoding mechanisms used in these systems. Discussing the molecular communication system model and looking at the unique characteristics of practical molecular communication systems and these chemical reactions and their effects on the communication performance. Finally, the book examines the point-to-point, broadcast, and multiple-access molecular channel and shows two promising application examples of the nanonetworks. The first application example is the body area nanonetworks used in nanomedicine. the second nanonetwork application example, i.e., NanoSensor Networks (NSNs) with Molecular Communication.