We describe the design and construction of ultrahigh capacity three-dimensional, 3D, optical storage devices that operate by two-photon absorption. The molecular systems and their properties that are used as two photon media for writing and one photon for accessing the stored information within the volume of the device are presented in some detail and the nonlinear two-photon absorption mechanism is briefly visited. The optical system and its components, which facilitated writing and reading, are also described and the bit density, bit error rate, store and access speeds, cycle times, and stability of the materials under various experimental conditions are also topics addressed in this review. The first ever storage of terabyte data in a removable storage disk is described in detail.
