  We review the existing theoretical approaches to the study of the observables
of hadronic atoms. In the relativistic perturbative approach based on the
Bethe-Salpeter equation and Chiral Perturbation Theory we derive the general
expression for the pi+pi- atom lifetime. The lowest-order correction to the
relativistic Deser-type formula for the atom width is predicted to be 6.1\pm
3.1 %.
