The key aspects of porous silicon and freestanding crystalline silicon nanoparticles for biological applications are discussed, with the following subjects being addressed: most simple and scalable synthesis procedures; importance of surface derivatization enhancing colloidal, chemical, photophysical stability, and capacity for reactive oxygen species photosensitization of the silicon nanomaterials in aqueous media, discussion of the origin of photoluminescence in the different SiNMs and the effect of surface derivatization; effects on the photophysical behavior of SiNMs upon the combination with transition metals or oxides of metals.