Here, chemical sensors made from porous Si are reviewed, with an emphasis on systems that harness photoluminescence and related energy-and charge-transfer. mechanisms available to porous Si-derived nanocrystallites Quenching of luminescence by molecular adsorbates involves harvesting of energy from a delocalized nanostructure that can be much larger than the molecule being sensed, providing a means to amplify the sensory event. The interaction of chemical species on the surface of porous Si can exert a pronounced influence on this process, and examples of some of the key chemical reactions that modify either the surface or the bulk properties of porous Si are presented. Sensors based on micron-scale and properties of Si particles are also discussed. Miniaturization to this size regime enables new applications, including imaging of cancerous tissues, indirect detection of reactive oxygen species (ROS), and controlled drug release. Examples of environmental and in vivo sensing systems enabled by porous Si are provided.