  Flexible long period moir\' e superlattices form in two-dimensional van der
Waals crystals containing layers that differ slightly in lattice constant or
orientation. In this Letter we show theoretically that isolated flat moir\' e
bands described by generalized triangular lattice Hubbard models are present in
twisted transition metal dichalcogenide heterobilayers. The hopping and
interaction strength parameters of the Hubbard model can be tuned by varying
the twist angle and the three-dimensional dielectric environment. When the flat
moir\'e bands are partially filled, candidate many-body ground states at some
special filling factors include spin-liquid states, quantum anomalous Hall
insulators and chiral $d$-wave superconductors.
