Quantum defect theory can be used to convert energy shifts of hadronic atom
s into zero-energy scattering lengths (and vice versa). The scattering leng
ths from quantum defect theory are in good agreement with those derived usi
ng all-order Deser-Trueman formulae (evaluated by the simple expedient of e
valuating the left-hand side of the effective range expansion exactly). Sca
ttering lengths derived from experimental energy shifts are presented for a
number of hadronic atoms.