This book is written for scientists involved in the calibration of viscometers. A detailed description for stepping up procedures to establish the viscosity scale and obtaining sets of master viscometers is given in the book. Uncertainty considerations for standard oils of known viscosity are presented. The modern viscometers based on principles of tuning fork, ultrasonic, PZT, plate waves, Love waves, micro-cantilever and vibration of optical fiber are discussed to inspire the reader to further research and to generate improved versions. The primary standard for viscosity is pure water. Measurements of its viscosity with accuracy/uncertainty achieved are described. The principles of rotational and oscillation viscometers are explained to enhance the knowledge in calibration work. Devices used for specific materials and viscosity in non SI units are discussed with respect to the need to correlate viscosity values obtained by various devices. The description of commercial viscometers meets the needs of the user.