Biosensors are required in a wide variety of applications for which existing technologies are inadequate. Recently, sensor elements with favorable properties have been made by engineering transmembrane protein pores. Analyte molecules modulate the ionic current passing through the engineered pores under a transmembrane potential. Stochastic sensing, which uses currents from single pores, is an especially attractive prospect. This approach yields both the concentration and identity of an analyte, the latter from its distinctive current signature. In one example, the bacterial pore-forming protein staphylococcal alpha-hemolysin (alpha HL) has been altered to permit the detection of divalent metal cations by using mutagenesis to place a cation binding site within the conductive pathway. In a second example. the hemolysin pore has been modified with cyclodextrins, which act as non-covalent molecular adapters, to allow the detection of a variety of small organic molecules. The great promise and wide applicability of stochastic sensing warrants efforts aimed at the development of a practicable device.