Mesoporous silica nanoparticles (MSNs) are introduced as chemically and thermally stable nanomaterials with well-defined and controllable morphology and porosity. It is shown that these particles possess external and internal surfaces that can be selectively functionalized with multiple organic and inorganic groups. On the basis of these characteristics, the biocompatibility of silica, and their efficient uptake by mammalian cells, MSNs are proposed as the basis of nanodevices for the controlled release of drugs and genes into living cells.