Terbium (Tb) is a soft, malleable, silvery white rare earth metal. It is primarily used in solid state devices, crystal stabiliser in fuel cells, sensor devices and in sonar systems. 70 % of the world supply of Tb is used in the television sets as green phosphors. The other practical applications of Tb include as an additive in magneto-restrictive alloys which changes length when subjected to a magnetic field. Gadolinium (Gd) is a metal ion with paramagnetic properties. This characteristic of Gd makes it particularly suitable for use as a contrast agent in magnetic resonance imaging (MRI). Gadolinium imaging has become an essential tool in the non-invasive evaluation of cardiovascular pathologies. Its current uses range from ischemia detection, viability assessment, cardiomyopathy study and non-invasive angiography, and its diagnostic and prognostic value has been demonstrated in multiple studies. This book discusses the chemical properties of both Tb and Gd, as well as the sources in which they can be found, and how they can be used.