  I review the relations between mass scales in various string theories and in
M-theory. I discuss physical motivations and possible consistent realizations
of large volume compactifications and low string scale. Large longitudinal
dimensions, seen by Standard Model particles, imply in general that string
theory is strongly coupled unless its tension is close to the compactification
scale. Weakly coupled, low-scale strings can in turn be realized only in the
presence of extra large transverse dimensions, seen through gravitational
interactions, or in the presence of infinitesimal string coupling. In the
former case, quantum gravity scale is also low, while in the latter,
gravitational and string interactions remain suppressed by the four-dimensional
Planck mass. There is one exception in this general rule, allowing for large
longitudinal dimensions without low string scale, when Standard Model is
embedded in a six-dimensional fixed-point theory described by a tensionless
string. Extra dimensions of size as large as TeV$^{-1}\simeq 10^{-16}$ cm are
motivated from the problem of supersymmetry breaking in string theory, while
TeV scale strings offer a solution to the gauge hierarchy problem, as an
alternative to softly broken supersymmetry or technicolor. I discuss these
problems in the context of the above mentioned string realizations, as well as
the main physical implications both in particle accelerators and in experiments
that measure gravity at sub-millimeter distances.
