Contrast agents for magnetic resonance imaging (MRI) currently used and approved by the Food and Drug Administration (FDA) of the United States of America are GdIII-based. Yet, GdIII complexes are only one class of contrast agents for MRI. The fundamentals described in this chapter are intended to assist with the design of future imaging agents for MRI. Ligands affect not only relaxivity but also thermodynamic stability and kinetic inertness of GdIII complexes, which in turn affect the safety profiles of contrast agents. Divided into four sections, this chapter describes syntheses and characterization of ligands and metal complexes as well as the determination of the stability and lability of metal complexes.