Enzyme-free, signal-amplified nucleic acid circuits utilize programmed assembly reactions between nucleic acid substrates to transduce a chemical input into an amplified detection signal. These circuits have shown great potential for developing biosensors for high-sensitivity and high-selectivity detection of varying targets including nucleic acids, small molecules and proteins in vitro and for high-contrast in situ visualization and imaging of these targets in tissues and living cells. We review the background of the enzyme-free, signal-amplified nucleic acid circuits, including their mechanism, significance, types and development. We also review current applications of these circuits for biosensors and bioimaging.