<p>At present, the output power density of nanogenerators (NGs) is insufficient to meet the substantial energy demands of many electronic devices, which significantly limits their further development in energy-harvesting applications. Therefore, optimizing the output performance of NGs is crucial for their advancement. This article focuses on the underlying principles and representative examples of nanogenerators, particularly those developed from nano- or microscale functional materials and utilized as nano- or micro-power sources. We introduce the fabrication and assembly of nanowire-based NGs, with an emphasis on materials such as zinc oxide (ZnO), gallium nitride (GaN), aluminum nitride (AlN), and Nylon-11 in terms of piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs). Additionally, we show fiber-based NGs incorporating materials such as poly(vinylidene fluoride) (PVDF), its copolymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), and poly(vinylidene fluoride-<i>co</i>-hexafluoropropylene) (P(VDF-HFP) in applications, such as yarns and textiles. Methods for surface modification and dielectric property control are discussed as strategies for optimizing the electrical energy output at the nano- or microscale. Furthermore, ZnO-based PENGs and cellulose-based TENGs are introduced as potential nano- or micro-power sources. Innovative applications such as flame-retardant textile TENGs, pyroelectric NGs, and implantable TENGs (iTENGs) for pacemakers are also discussed.</p> Graphical abstract <p>Nano- or microscale functional materials and optimized surface modification/control are the key to achieving nanogenerators as nano- or micro-power sources for next-generation Internet of Things (IoT)/wearable electronics.</p> <p></p>