The High Altitude Lensing Observatory (HALO) is a balloon-borne telescope and camera system that will constrain the properties of the dominant components of the Universe. The current concordance model in cosmology holds that the dual phenomena of dark matter and dark energy collectively comprise over 95% of the mass/energy of the Universe, but the properties of this dark sector are poorly constrained. Weak gravitational lensing, whereby the images of background galaxies are slightly distorted by foreground dark matter, is an ideal probe of dark matter and dark energy. HALO will perform a 200-1000 square degree weak lensing survey measuring the shapes of 15-20 galaxies per square arcminute while flying at 35 km. By observing at this altitude, above more than 99% of the atmosphere, HALO will be able to achieve near space-quality data, avoiding the seeing associated with ground-based telescopes. HALO will thus perform a survey that is statistically competitive to ground-based surveys in the same time frame, but with quality and systematics control close to those of a space-based survey. HALO thus represents an important intermediate step in high-precision weak lensing measurements between surveys done with the Hubble Space Telescope and ambitious space surveys planned for late in the next decade. I will present the science justification for HALO, the baseline design, and proposed survey strategy.