Nanosized mesoporous silica particles with high colloidal stability attract growing attention as drug delivery systems for targeted cancer treatment and as bioimaging devices. This Perspective describes recent breakthroughs in mesoporous silica nanoparticle design to demonstrate their high potential as multifunctional drug delivery nanocarriers. These types of nanoparticles can feature a well-defined and tunable porosity at the nanometer scale, high loading capacity, and multiple functionality for targeting and entering different types of cells. We focus on the requirements for an efficient stimuli-responsive and thus controllable release of cargo into cancer cells and discuss design principles for smart and autonomous nanocarrier systems. Mesoporous silica nanoparticles are viewed as a promising and flexible platform for numerous biomedical applications.