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The rapid expansion of nanotechnology promises to have great benefits for society, yet there is increasing concern that human and environmental exposure to engineered nanomaterials may result in significant adverse effects. That is why the field of nanotoxicology – dealing with effects and potential risks of particulate structures <100 nm in size – has emerged, growing significantly over the past decade from long-standing foundations of well established knowledge on the toxicology of fibrous and non-fibrous particles and the interactions of viruses with cells. This review places nanoparticles in the context of conventional particle toxicology and so includes references to other types of particles, such as silica and asbestos, which have been extensively studied and can provide useful lessons relevant to newly engineered nanoparticles (NP). Discoveries of nanoparticle-specific concepts of toxicology related to their small size and large specific surface area go back to the early parts of the past century, although the distinctive biological effects and kinetics of NP were not recognized until the last decade of the past century. Today, the propensity of NP to cross cell barriers, enter cells and interact with subcellular structures is well established, as is the induction of oxidative stress as a major mechanism of nanoparticle effects. In addition to the significance of small size and surface area of NP, uncovering the impact of many other physico-chemical characteristics – in particular NP surface properties – for initiating effects in the mammalian organism and the environment is now an active area of research. The article aims to cover hazards relevant to humans, provides an introduction to some of the newly emerging literature on fate and behavior of NP in the environment, as well as describing their ecotoxicology in a variety of species. Major milestones in the research leading to our present understanding of nanotoxicology and the potential risks of NP to humans and the environment are summarized. These risks are likely to be different for different nanomaterials, ranging from perceived and very low for most, to real and very high for some. There are many questions that remain to be addressed, and we foresee for the future a continuing extended research in nanotoxicology. A full understanding of the hazard of NP will make a major contribution to the risk assessment that is so urgently needed to ensure that products that utilize NP are made safely, are exploited to their full potential and then disposed of safely.