Nature has long used nucleic acid aptamers and enzymes for regulatory activities, such as the recently discovered “riboswitches” involved in gene expression. The existence of a large array of natural and artificial functional nucleic acids has generated tremendous enthusiasm and new opportunities for molecular scientists from diverse disciplines to devise new concepts and real applications that take advantage of those nucleic acids for sensing and other analytical applications. This book provides a timely and comprehensive overview of recent advances in the field, from leading experts in biology, chemistry, and engineering. A variety of topics are covered, from fundamentals of functional nucleic acids, to their applications as sensors, to nanotechnologies; as well as integration of functional nucleic acids into practical analytical systems.