Nowadays biological sensing is representing a very active area of research due to the many possible applications in
environmental control, food testing, pharmaceutical screening, and genetic analysis. A particular need exists for
miniature biosensors for the detection of proteins, DNA, toxins and, more generally, infectious agents. Several optical
techniques have proven to be quite effective, and biophotonics is a continuously growing discipline. Here we provide a
quick overview of the recent progresses in the development of optical biosensors based on resonant cavities constituted
by dielectric microspheres, where light propagation occurs through whispering-gallery modes (WGMs). The effect of
any perturbation to the optical resonance structure of a microsphere is such that a very high sensitivity can be achieved.