Rare-earth-metal (Ln) alkyl and aryl compounds feature highly reactive, thermally labile [Ln–C] s-bonds which result in rapid and violent decomposition in the presence of air and moisture. Nevertheless, such [Ln–C] moieties display important intermediate species in numerous organic transformations. Reagents containing [Ln–C] bonds are deliberately generated in situ like Imamoto-type reagents Ln(III)Cl3/RLi (routinely at low temperatures) or “heavy” lanthanide-Grignard compounds RLnX. Samarium(III)-alkyl species are supposed intermediates of SmI2-promoted Barbier-type carbon– carbon coupling reactions. Alkyl/aryl ligand exchange at Ln(III) centers has been identified as the crucial step of lanthanide–halogenexchange reactions. In the meantime, several such mixed alkyl/halogenido complexes, devoid of an ancillary ligand, could be isolated and scrutinized with regard to structure and reactivity. More recently, Ln(III)-alkylidene complexes could be accessed, structurally authenticated and successfully employed in Tebbe-like olefination reactions of ketones.