The complexation by aminopolycarboxylate ligands has been a major area of interest in lanthanide chemistry for almost five decades. The number of donor sites and the variety of structures available in these ligands has provided opportunities to study a number of parameters in lanthanide complexation. It can be shown that the entropy changes are related to the number of carboxylate donors while the enthalpy changes depend on both carboxylate and nitrogen donors. The Delta H-Delta S compensation indicates the importance of hydration effects on these terms, in contrast to the Delta G of formation which reflects, predominantly, the cation-ligand interactions. Variation of the length of the alkyl chains has shown that 5-membered rings are strongly favored and that the O-Ln-N rings are more sensitive to size than those of N-Ln-N.