Chapter 5: is a brief look into the many and diverse applications of photonics and the essential principles underlying them. Many are familiar – displays and lighting being the most prevalent.Imaging, especially in the era of the mobile phone, is an everyday operation and astronomical telescopes are regulars in publically presented the science and technology reports.Optical communications has also become taken for granted – optical fibre enables the internet!Less familiar perhaps are the use of ultra-violet and other sources in photo-therapy, lasers in surgery and industrial machining and, increasingly important, in environmental analysis and monitoring.And many high street opticians now have subtle probes to assess retinal health based on three dimensional tomographic probes.Photonics has indeed crept into our everyday environment – and will continue to become increasing important as an enabler of technological advances.