The exploration of nanomaterialsNanomaterials has been one of the most promising scientific innovations of the twenty-first century. Due to their unique and novel characteristics in reduced dimensions, conjoined with recent progress in material synthesis and device fabrication, remarkable new applications continue to evolve. Nanotechnology, a resulting multidisciplinary field, has demonstrated significant progress in health and medicine, safety, security, and sustainability, thus leading to a better global economy. New discipline also means new rules of engagement, regulation, and concerns. International organizations are working together to organize the subject, in terms of definitions, nomenclature, tests, standards, and prevention measures as a regulatory framework. However, technology generally progresses faster than regulations, and hence unregulated products continue to invade the market, without proper certification or even knowledge regarding their safety or their possible risks. Often, consumers are uninformed of the new technology applied to certain products. Researchers and consumers are starting to share a growing concern about the possible impact of nanotechnologies on human health and the environment, mostly due to the unknown behavior and effects of nanoparticles. Since engineered nanoparticles are unknown to Nature, in general, and the human body, in particular, their toxicology is largely unknown. In most cases, the reaction to cell sensors is not well understood and classical toxicology does not seem to work properly, for the simple reason that nanoparticles, unlike bulk materials, can enter cells and interact directly with the tiniest biological organelles like mitochondria, ribosomes, endosomes, or entities such as proteins, RNARNA, and DNADeoxyribonucleic Acid (DNA). Many nanoparticles are crystalline structures and are not bioresorbable, i.e., physically, chemically, or biologically not degradable either in the extracellular matrix or inside the cells. From a securitySecurity standpoint, it presents a major concern since it offers an ill opportunity for someone with nefarious intentions. In view of this dual use of nanomaterialsNanomaterials, this chapter extends an in-depth discussion of opportunities and risks presented by nanoparticles.