Semiconductor nanowires represent a novel class of nanostructured materials with a wide range of future applications from molecular electronics to biotechnology. Using appropriate fabrication procedures, our group has previously demonstrated that field-effect transistors (FETs) made from p-type Si nanowires possess electronic characteristics exceeding that of conventional planar devices. This outstanding electronic property makes nanowire FETs ideal transducers in a sensor system with label-free, real-time detection capability. Furthermore, sensors made from Si nanowires offer additional advantages over other type of sensors including the ease to differentially modify many nanowires for multiplexed sensing, the potential to be very small and inexpensive, and most importantly the unparalleled extreme sensitivity to the point where single molecule detection is possible. With successful chemical modification to covalently immobilize biological receptors onto the surface of nanowires, we showed that a nanowire FET can be configured into a nano-scale sensor and the binding of charged ligands to the receptors generates specific electrical responses in a quantitative manner. We first applied this strategy to develop a sensitive detector for prostate cancer by measuring the levels of PSA, a marker for prostate cancer. The sensor was shown to detect PSA as low as 0.025 pg/ml (7fM). In addition to medical diagnostics, the combined advantages of label-free detection and extreme sensitivity offer a unique opportunity to configure the nanowire sensors into a drug discovery platform. Using Abl kinase/ATP/Gleevec as a model pathological system (in chronic myeloid leukemia), we have demonstrated the possibility to visualize drug action, or small molecule/protein interactions in real time. Lastly, because of the high sensitivity inherent to the nanowire sensors, individual binding/unbinding events of single molecules can be resolved electrically.