  We derive a low-energy theory for edge states in transition metal
dichalcogenide monolayers for a two-band $\bm{kp}$-Hamiltonian in case of
uncoupled valleys. In the absence of spin-orbit interaction at the edge, these
states possess a linear dispersion described by a single phenomenological
parameter characterizing the edge structure. Depending on the sign of the
parameter, the edge state spectrum can either cross the band gap or lie outside
of it. In the first case, the presence of edge states leads to resonant
enhancement of the second harmonic generation at frequencies about the half of
the band gap, in agreement with recent experiments. The value of the
phenomenological boundary parameter is extracted from the resonance frequency
position.
