wires [5] and split-ring resonators (SRRs), [6] respectively. In 2001, the first experimental observation of negative refraction was presented by combining the metal wire and SRR structures, [7] which have intrigued worldwide research interests among scientific and engineering communities and led to many exotic phenomena, such as inverse Doppler effect, [8] reversed Cherenkov radiation, [9] and perfect lens. [10] At the early time of metamaterials, most researches are focused on the realization of double-negative materials using various metallic structures. [1,3,4,7,9] These doublenegative materials suffer from serious insertion loss due to the lossy nature near the resonant frequency, which had long hindered their applications from the microwave to visible light spectra. Later, researchers noticed that the permittivity and permeability of an artificial material can be arbitrarily designed from negative value to positive value by selecting the nonresonant frequencies or adopting nonresonant structures. This extends the constituent parameters of metamaterials from negative ranges to the entire μ-ε space, and changed the way people study and design metamaterials. Owing to its lower loss and much easier experimental implementation, these generalized metamaterials have brought a series of new physical phenomena and interesting devices, including invisibility cloaks, [11][12][13] illusion optics devices, [14][15][16] EM black hole, [17] metamaterial lens antenna, [18] and super-resolution imaging. [19] Before the emergence of metasurfaces, metamaterials were commonly designed in the 3D form with spatially varied refractive index, in which light passing through different optical paths experience different phase delays, leading to the desired wavefronts of the outgoing waves. Obviously, such metamaterials with isotropic, anisotropic, and even tensor-form constituent parameters pose great challenges for fabrications, especially at higher frequencies where micro-and nano-3D fabrication techniques are far from maturing. [20,21] In addition, the bulky sizes and heavy weights of 3D metamaterials have become major obstacle for applications in compact devices. To overcome the above disadvantages, 3D metamaterials have been degraded into 2D forms, in which the direction of wave propagation is squeezed down to almost zero thickness, leading to the wellknown metasurfaces or metafilms. [22] As metasurfaces can be treated as an infinitely thin sheet with certain effective electric As a digital version of metamaterials, coding and programmable metamaterials have experienced rapid development since they were initially proposed in 2014. Unlike conventional metamaterials that are characterized by the sophisticated effective medium theory, coding metamaterials are described in a much simpler manner with binary codes, which builds up a bridge between the physical world and the digital world. In this article, the development of coding and programmable metamaterials in the past three years is reviewed, focusing primarily on the basic conce...