Lipid nanoparticles, such as solid lipid nanoparticles and nanostructured lipid carriers, are drug delivery systems in which solid lipids are dispersed in an aqueous phase stabilized by a surfactant layer. The great interest in these nanocarriers in the latest years is due to the biocompatible lipid matrix, associated with the potential for sustained drug release, and easy transposition to the industrial scale. Moreover, these lipid systems present the ability to prevent drug degradation, and to enhance cell uptake, usually increasing drug efficacy. This chapter will provide an overview of the recent literature on solid lipid nanoparticles and nanostructured lipid carriers for drug delivery applications. Thus, some background information on the origins, composition, characterization parameters and biological applications of these nanocarrier systems will be presented.