The introduction of the ISO 9000 quality standard has resulted in renewed interest and pressure on industry to strengthen their quality and metrology standards. This is resulting in new interest and vigour in calibration in manufacturing industries and calibration laboratories world-wide. To meet this renewed interest, this text provides information on mass metrology. It aims to highlight the principles of physics involved and the technology needed to accurately measure the density of solids and liquids to high precision to meet the increasing demands on the metrology industry. Starting with a look at national and international density standards the book proceeds to discuss the variety methods used to accurately measure solid and liquid density, comparing and contrasting these techniques and providing a discussion of thermal dilation of liquids. Interferometers used in dimensional measurements of solid based density standards and corrections applicable due to finite aperture, phase change due to reflection and ringing and special methods for density determination are included. The final chapters detail specific points of relevance to density measurements and hydrometry for materials commonly used in industry.