Biosensors are analytical devices which combine a biologically sensitive element with a physical or chemical transducer to selectively and quantitatively detect the presence of specific compounds in a given external environment. The interaction of the compound (the analyte) and the biological element generates a signal (electrical, optical or mechanical) which is measured by an optical or electronic system, usually following amplification. Increasingly, biosensors are exploiting state-of-the-art technologies such as protein engineering and advanced microelectronics to offer easy-to-use, widely applicable devices which are compact as well as cheap.