In recent years, nanoparticles have received increasing attention in research and technology, including a variety of practical applications. The bioactivity appears to be related to the small particle size, in addition to inherent chemical activity as electron transfer (ET) agents, generators of reactive oxygen species (ROS) with subsequent oxidative stress (OS), and as antioxidants (AOs). The mechanism of toxicity, therapeutic action, and AO property is addressed based on the ET-ROS-OS approach. There are several main classes of ET functionalities, namely, quinones (or phenolic precursors), metal compounds, aromatic nitro compounds (or reduction products), and imine or iminium species. Most of the nanospecies fall within the metal category. Cell signaling is also discussed. This review discusses recent developments based on ET-ROS-OS-AO framework.