We develop a thermodynamically consistent continuum theory for the stu
dy of solid-solid phase transitions where deformation dominates heat a
nd mass transfer. We use an order parameter to characterize the notion
of phase and identify phase interfaces with thin transition zones wit
hin which the strain and order parameter exhibit large gradients. To m
odel die growth of one phase at the expense of another we introduce co
nfigurational forces, subject to their own balance, that work against
changes in the order parameter. By studying traveling waves and the, a
symptotics of a transition layer, we establish connections between our
theory and more standard approaches that identify phase interfaces wi
th surfaces of strain discontinuity.