<p>Nanogenerators are an emerging technology for energy harvesting, which is based on diverse physical effects such as piezoelectric, pyroelectric, triboelectric, and tribovoltaic mechanisms to convert irregular high-entropy energy into electricity. Attributed to their merits of portability, flexibility, and wide applicability, nanogenerators have garnered significant attention and shown potential applications in fields, such as the Internet of Things, self-powered sensors, and artificial intelligence. This article provides a comprehensive review of the advancements in nanogenerator technology, including the fundamental principles, core mechanisms, and output characteristics of piezoelectric, pyroelectric, triboelectric, and tribovoltaic nanogenerators. Additionally, we also compare and discuss the future research opportunities of these different nanogenerator mechanisms.</p> Graphical abstract <p></p>