<p>Electron-deficient building blocks are fundamental components in the construction of n-type conjugated polymers, which play a pivotal role in a broad range of organic optoelectronic devices. Over the past decades, considerable efforts have been devoted to the design and synthesis of novel electron-withdrawing units to expand the structural diversity and enhance the performance of conjugated polymers. This review systematically summarizes recent developments in electron-deficient moieties, including fused-imide cores, amide-based cores and quinoids. In addition, representative synthetic strategies and electronic structures are discussed. Particular attention is paid to molecular design principles, such as backbone planarity, intramolecular interactions, and side-chain engineering that contribute to the n-type performance in organic thin-film transistors (OTFTs) and organic thermoelectrics (OTE). The review summarizes with a perspective on future directions in the development of n-type conjugated polymers through innovative building block design.</p>