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Organic or inorganic nanoparticles, introduced into the organism intentionally for their therapeutic properties or unintentionally as pollutants, can persist there over the long term within cellular compartments and undergo numerous transformations that modify their physical properties as well as their morphology and tissue localisation. The links between the biotransformations (degradation, elimination, recycling by endogenous proteins, recrystallisation, biomineralisation) carried out by the biological environment on nanoparticles, their functionalities for diagnostic or therapeutic use and their potential short- or long-term toxicity remain poorly understood. We will give several examples of intracellular biotransformations of metallic (iron oxide, gold, silver, titanium) or carbonaceous (carbon nanotubes) nanoparticles that show unexpected effects and shed light on the life cycle of nanoparticles. In particular, we will illustrate the essential contribution of the various electron microscopy or nanometrology techniques for monitoring the fate of these nanostructures in a biological environment. Image of a lysosome observed by transmission electron microscopy, a method capable of revealing the form of nanoparticles within cells. The degradation products are composed of gold crystals 2 nm in diameter, self-organised into nanosheets. The undegraded nanoparticles are in red and the degradation products in orange(c) Alice Balfourier, MSC laboratory (CNRS/University of Paris) and Christine Péchoux-Longin (MIMA2 platform, INRA).