The classical and the quantum massive string model based on a modified
BDHP action is analyzed in the range of dimensions 1 < d < 25. The di
scussion concerning classical theory includes a formulation of the geo
metrical variational principle, a phase-space description of the two-d
imensional dynamics, and a detailed analysis of the target space geome
try of classical solutions. The model is quantized using the ''old'' c
ovariant method. In particular an appropriate construction of DDF oper
ators is given and the no-ghost theorem is proved. For a critical valu
e of one of the free parameters of the model the quantum theory acquir
es an extra symmetry not present on the classical level, In this case
the quantum model is equivalent to the non-critical Polyakov string an
d to the old Fairlie-Chodos-Thorn massive string.