Dusty star-forming galaxies (DSFGs) host the most prodigious stellar nurseries in the Universe, with star formation rates typically between 200-2000 Msun/yr. Though these galaxies are rare in our local universe, they were orders of magnitude more abundant at z~2-5, contributing significantly to the buildup of the Universe's stellar mass and the formation of high-mass galaxies. Due to dust obscuration, DSFGs are invisible to optical and ultraviolet surveys. However, recent large-area millimeter/submillimeter surveys have become available and proven to be efficient for selecting rare (<0.1 per sq. deg.), strong gravitationally lensed DSFGs. In this talk, I will discuss the large (~100) sample of these sources discovered by the South Pole Telescope. These sources were selected by their extreme mm flux and span the redshift range from 1.8< z < 6.9 with a median of z=3.9, extending into the epoch of re-ionization. I will also discuss the ways in which gravitational lensing acts as a cosmic microscope, dramatically reducing the integration time required to detect these sources, making weak spectral lines accessible, and increasing the spatial resolving power of our telescopes. These high-redshift strongly lensed galaxies are important tools in assessing the condition of the interstellar medium in the early Universe and can even place limits on dark matter substructure.
