Airplanes in clouds, submarines in murky water, and cancerous tumors in breast tissue all have one thing in common, they are objects hidden in highly scattering media. Optical imaging through such media remains one of the most challenging problems in science and engineering, but the advent of ultrafast lasers and detectors, coupled with a range of time-resolved techniques, has led to recent breakthroughs. Seeing through highly scattering media, normally absorbing the vision by image blurring, is desirable in many contexts. Fog, aerosols, and dust can strongly impair the vision in the atmosphere from Rayleigh and Mie scattering. At high particle concentrations, multiple scattering becomes dominant. The infrared spectral region, where scattering by fog and dust becomes much smaller, is used in many viewing systems.