<p>Multicomponent reactions (MCRs) have emerged as powerful and convergent tools for the rapid assembly of structurally complex and biologically significant heterocycles. Among the wide spectrum of nitrogen-containing frameworks, three- and four-membered systems such as aziridines, azetidines, and azetidinone occupy a privileged position due to their intrinsic ring strain, conformational rigidity, and broad pharmacological relevance. This review highlights the conceptual evolution and mechanistic diversity of MCR-mediated approaches to these strained heterocycles, encompassing transition-metal-catalyzed, organocatalytic, radical, and photochemical strategies. Particular emphasis is placed on recent developments including Cu-catalyzed alkyne–azide–carbodiimide coupling, visible-light-induced Wolff rearrangements, dual Cu/photoredox-catalyzed radical relay processes, and sustainable metal-free protocols employing silica and biopolymeric chitosan. The review also examines the role of post-MCR cyclizations, ABB-type insertions, and Ugi cascade reactions in expanding molecular diversity. Mechanistic insights into ketene, ketenimine, and radical intermediates are discussed alongside examples of ultrasound- and nanocatalyst-assisted MCRs that exemplify the integration of green chemistry principles. Collectively, these methodologies demonstrate how multicomponent design enables the efficient synthesis of stereochemically rich, pharmacologically potent small-ring heterocycles. The review concludes with current challenges in stereocontrol, scalability, safety, and sustainability, and outlines future perspectives in flow photochemistry, computational optimization, and hybrid catalysis for precision-controlled heterocyclic synthesis.</p>