Flat optical elements including a multi-level metasurface stack having two or more metasurface levels. Each metasurface level includes an arrangement of nanostructures, or protrusions, of one or more optically transmissive materials. A metasurface level may further include another optically transmissive material between the nanostructures, achieving a desired index contrast. Another metasurface level including additional nanostructures may be over a planar surface of this additional transmissive material. Another optically transmissive material may be between the additional nanostructures. This architecture may be followed for any number of levels, (e.g., a bi-layer, tri-layer, etc.). Each metasurface within the multi-level metasurface structure may be tuned to a particular optical wavelength. Such a multi-level metasurface may have greater bandwidth and/or achieve higher optical efficiency for a given band than a single metasurface. Molding techniques may be employed in metasurface fabrication to reduce the starting material cost and fabrication cost.