Proteins generally execute the key physiological activities required for normal growth and homeostasis. As such, many different classes of proteins, including proteases, kinases, cellular receptors and signalling proteins, represent attractive targets for diagnosis or therapy. Aptamers are small nucleic acid molecules that function as direct protein inhibitors, much like monoclonal antibodies. After a decade of intensive research, technology development and initial clinical evaluation, aptamers have now demonstrated broad potential as direct protein ligands and inhibitors, and thus represent an exciting class of compounds for the development of new therapeutic and diagnostic agents. This review will discuss the basic properties and isolation of aptamers, their use in animals and the clinic, and describe an exciting recent advance in the development of antidotes to certain aptamers, which will add a repertoire of new agents with regulatable activity for clinical use.