This paper reviews the ways in which micro and nano sensors have evolved within biology and medicine. The target measurands include an ever-increasing number of simple and complex molecules, physical quantities, and electrical and magnetic phenomena. Micro sensors based on electrochemical, acoustic, piezoelectric and optical principles are contributing to clinical care of patients who may benefit from the continuous monitoring of critical variables in intensive care or from the ability to perform convenient self-monitoring during normal daily life. Sensors constructed on the nano-scale are now emerging, especially for complex bio-molecules such as DNA. These are strengthening basic research, for example in the study of genetic factors in disease and for discovery of new drugs. Scanning probe technology and nano optics, including surface enhanced Raman spectroscopy, play important roles in these developments. Sensor science and technology has gained significant benefits through inspiration arising from biological sensory systems. This includes the sense of olfaction, which has led to the artificial nose, and the sense of vision that has been emulated in several versions of the artificial retina. The impact of micro and nano sensors on fundamental understanding in biomedicine and on clinical diagnosis and care are highlighted.