The ease of fabricating high quality photonic crystals from porous silicon and its biocompatibility have inspired the conception of various biosensing schemes using this material. However, the instability of porous silicon has significantly slowed progress in this area. Here we discuss the potential of different porous silicon photonic crystals for biosensing in the context of its surface chemistry and nanostructure, both of which need to be optimized to obtain sensitive and stable devices. Of particular promise are recent approaches that use porous silicon as sensors for enzymatic activity, for cell capture and concentration devices.