This review considers the most recent experimental developments in the magnetic anisotropy of mono-nuclear transition metal complexes in various coordination environments. Two- to eight-coordinated complexes are considered on the basis of low temperature evaluations of their anisotropy by magnetic studies. In particular, we focus on studies performed to determine slow magnetic relaxation processes,i.e., single-molecule magnet (SMM) behavior. We discuss the effects of electronic configurations and symmetry on magnetic anisotropy and associated dynamic magnetic properties. The use of well characterized mononuclear complexes with strong magnetic anisotropy as building units for the preparation of polynuclear SMMs and single chain magnets (SCMs), is addressed in a special section.