Communications at Nanoscale.- Why do we need communication at nanoscale?.- Frontier applications of nanoscale communications.- Nanoscale Communications in Nature.- Why does the Nature need nanoscale communication?.- Nanoscale communications in Nature.- Adaptation and Inspiration from the Nature: Towards Molecular Communication Systems and Applications.- Active Molecular Communication.- Passive Molecular Communication.- Gap Junction Molecular Communication.- The Existing Molecular Communication Systems and Applications.- Molecular Transmitters.- Molecular Receivers.- Molecular Encoding Mechanisms.- Molecular Decoding Mechanisms.- Molecular Communication Applications.- The Envisioned Molecular Communication Systems and Applications.- Feasibility of the Envisioned Molecular Communication Systems.- Information Theoretical Foundations of Molecular Communication.- Molecular Channel.- Molecular Channel Characteristics.- Noise in the Molecular Communication Channel.- Attenuation in the Molecular Communication Channel.- Capacity of the Molecular Communication Channel.- Optimal Encoding and Decoding for Molecular Communication.- Molecular Modulation.- Molecular Nanonetworks.- Networking in Molecular Nanonetworks.- Molecular Medium Access Control.- Molecular Routing.- Molecular Transport.- Molecular Addressing.- Application Example: Body Area Nanonetworks with Molecular Communication.- Application Example: Nanosensor Networks with Molecular Communication.