Radiometry is a fascinating, fast growing research area, and there are many interesting real life applications. This book is intended to provide readers the theoretical background of radiometry, a resource of the latest radiometry technology, as well as the latest research in radiometry. It is aimed at university/college students, researchers and engineers. It assumes readers have basic knowledge and skills concerning electronics, physics and mathematics at the university level. This book is divided into three parts. Part I is the Introduction to Radiometry, which includes the theoretical background of radiometry, radiometry sources, radiometry detectors, and radiometry optical systems. This part also includes the latest technologies available, such as different Quantum Cascade Lasers, wavelength tuneable detectors, thermal electric cooled and Stirling cooled detectors, multispectral and hyperspectral thermal cameras, high resolution and high speed thermal cameras, and various radiometry optical detection systems. Part II is called the Advances in Radiometry Research, which contains the development of the latest research in areas of biomedical applications, industrial applications, non-destructive testing, astronomy and environmental applications. This is the core part of the book, and provides a review of the latest research trends in radiometry in different application areas. It also includes a chapter on prototyping low cost radiometry devices, which provides a list of low cost lasers and detectors, low cost and compact thermal cameras, low cost optics, low cost PCB making, and finally low cost 3D printers and CNC machines. Part III is the Appendices, which includes symbols used in the book, some MATLAB example codes including least squares fitting and the latest deep learning GoogLeNet, the introduction to WolframAlpha, a list of optical, infrared and laser components suppliers, and radiometry books. This book can be used as a textbook as well as a background reading textbook, or as a resource book.