The resonant tunnelling diode (RTD) is a semiconductor device that can act as the highest speed electronic amplifier and oscillator (bandwidth >1 THz). It is made using ultrathin (<10 nm) layers of semiconductor alloy that can be easily integrated with photonic devices such photodetectors and lasers. Incorporating the ultrahigh speed RTD amplifier gives the photonic devices new capabilities that are described in the book—as are exciting new applications made possible by integrating the RTD with photonic devices such as neuromorphic (brain-like) photonic computing. Some of these systems provide real-world applications of concepts from nonlinear dynamical theory—concepts including excitability, chaos and synchronisation.