As a consequence of the renewed interest of β–lactams in organic and medicinal chemistry, research in this field is constantly providing new appealing advances. This chapter focuses on synthetic aspects related to the generation of this four-membered heterocyclic ring, published since 2000, with special emphasis on stereoselective synthetic approaches. The compendium deals with methods for achieving diastereo- and enantioselectivity during [2+2] cycloadditions, with the Staudinger reaction still as first choice, but also important achievements have been published for the imine-enolate condensation, the carbonylative cycloaddition and the Kinugasa reaction. Significant success has also been accomplished in the synthesis of innovative 2-azetidinones through the formation of a single bond. In this respect, the C3-C4 bond formation, through intramolecular cyclization of linear epoxy or halo derivatives and the carbenoid C-H insertion are among the most effectively exploited procedures. In addition, adaptation of most procedures to solid-phase methodologies has facilitated the generation of molecular diversity based on the β–lactam skeleton.