Even though Ziegler catalysts have been known for almost half a century, rare earth metals (Ln), particularly neodymium (Nd)-based Ziegler catalyst systems, only came into the focus of industrial and academic research well afterthelargescaleapplicationoftitanium,cobaltandnickelcatalystsystems. Asadirectconsequenceofthelaterecognitionofthetechnologicalpotentialof rareearthmetalZieglercatalysts,thesesystemshaveattractedmuchattention. Considerable progress has been made in this ?eld as a result of intensive workperformed during the last fewyears. Worthmentioning is thestructural identi?cation of a variety of Ln/Al heterobimetallic complexes and the role of alkyl aluminum cocatalysts in molar mass control. Furthermore, a deeper understanding ofthe polymerization mechanism, suchas theliving character of neodymium-catalyzed diene polymerization associated with the reversible transfer of living polymer chains between Nd and Al, was revealed quite - cently. In spite of the vast number of patents and publications mainly issued duringthelastdecade,acomprehensivereviewthatcoversthescienti?caswell as the patent literature has been missing until now. In this volume we try to review the available literature by two independent approaches to Nd and Ln-catalyzed diene polymerizations. In the ?rst part of thevolume,whichisentitled"Neodymium-Based Ziegler/NattaCatalystsand their Application in Diene Polymerization", a polymer chemist's view is given with strong emphasis on Nd-based catalyst systems. Also technological and industrialaspectsofNd-catalyzeddienepolymerizationsareaddressed.Inthe secondpartofthevolume,whichisentitled"Rare-EarthMetalsandAluminum Getting Close in Ziegler-type Organometallics", a more organometallic p- spective isgivenandLn-based catalystsystems areaddressed. Bythesynopsis of these different perspectives, the reader will comprehend the complexity of Ln-based Ziegler catalyst systems and their application to the polymerization of dienes. This volume also gives a description ofthe evolution in Nd-catalyzed po- merization of dienes from the early works to the current state of the art.