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Electronic energy transfer is a crucial photophysical process, since it is at the basis of essential natural phenomena such as photosynthesis, as well as of widely spread artificial molecular devices; moreover, it is a very valuable tool for measuring distances at the nanometer level. For artificial systems, the possibility to take profit of energy transfer processes has enormously increased with the advent of supramolecular chemistry,1 and a new additional boost is expected to be observed with the very rapid growth of nanotechnology, in many different fields, ranging from medical diagnostics to solar energy conversion. In the context of nanotechnology, a great and increasing interest is devoted to nanoparticles.2 Nanoparticles can be made of many different materials, including polymers, metals, semiconductors, or a combination of them. Their versatility and different properties have already gained them many industrial applications in a wide range of fields, such as electronics, medicine and material sciences. One of the cut-edge fields in nanoparticle research is the enhancement of biological imaging for medical diagnostics and drug delivery. In this article, we will focus our attention on the potentialities offered by the use of energy transfer processes in dye-doped or dye-coated silica nanoparticles (DDSN or DCSN respectively),3-10 which conjugate a simple and low-cost preparation with the possibility to obtain sophisticated, but robust, multifunctional systems, especially when they are designed as ‘onion-like’, multilayer structures. The design of these systems is not trivial and it has to be carefully studied to be able to foresee the photophysical characteristics of the resulting materials. On the other hand, despite the complexity of these systems, their synthesis is relatively simple and very versatile. These features make them a very powerful tool to obtain very complex and precious functions from low cost and easy to prepare nanoobjects. In the next paragraphs we will present the most common synthetic strategies to obtain dye doped and dye covered silica nanoparticles and discuss many examples, but we would like to start introducing the energy transfer process and its great scientific and applicative importance.