One of the big challenges of medicine today is to deliver drugs specifically to defected cells. Nanoparticulate drug carriers have the potential to answer to this call, as nanoparticles can cross physiological barriers and access different tissues, and also be provided in a targetable form aimed at enhancing cell specificity of the carrier. Recent developments within material science and strong collaborative efforts crossing disciplinary borders have highlighted the potential of mesoporous silica nanoparticles (MSNs) for such targeted drug delivery. Here we outline recent advances which in this sense push MSNs to the forefront of drug delivery development. Relatively straightforward inside-out tuning of the vehicles, high flexibility, and potential for sophisticated release mechanisms make these nanostructures promising candidates for targeted drug delivery such as 'smart' cancer therapies. Moreover, due to the large surface area and the controllable surface functionality of MSNs, they can be controllably loaded with large amounts of drugs and coupled to homing molecules to facilitate active targeting, simultaneously carrying traceable (fluorescent or magnetically active) modalities, also making them highly interesting as theragnostic agents. However, the increased relative surface area and small size, and flexible surface functionalization which is beneficially exploited in nanomedicine, consequently also includes potential risks in their interactions with biological systems. Therefore, we also discuss some safety issues regarding MSNs and highlight how different features of the drug delivery platform influence their behaviour in a biological setting. Addressing these burning questions will facilitate the application of MSNs in nanomedicine. © The Royal Society of Chemistry 2010.