This chapter focuses on biosensor types, their detection limits, analysis times, and the diseases they are suitable for detecting. In addition, as nanomaterials are an effective means of producing small‐scale diagnostic devices, nanostructures have been commonly employed in biosensor design. Consequently, a section is devoted to the types of nanomaterials currently under use in biosensor design. Biosensors can be classified according to their recognition element (e.g., enzymes, antibodies, nucleic acids), output type (e.g., optical, electrical, mechanical), detection principle (e.g., surface plasmon resonance (SPR) based, surface‐enhanced Raman spectroscopy (SERS) based, quartz crystal microbalance (QCM) based), or intended use (in vivo or ex vivo). These factors all play vital roles in determining the sensitivity and selectivity of a biosensor and are considered separately.