Tamm states of light are lossless interface modes decaying exponentially in the surrounding media. We show that they can be formed at the boundary between two periodical dielectric structures, one having a period close to the wavelength of light and another one having a period close to the double of the wavelength. The order of layers at the interface has a crucial effect on the Tamm states. The in-plane dispersion of these states is parabolic with effective masses slightly different for TE and TM polarizations, both of the order of 10 −5 of the free electron mass. DOI: 10.1103/PhysRevB.72.233102 PACS number͑s͒: 78.68.ϩm, 42.70.Qs, 41.20.Jb, 42.25.Lc Surface waves are a specific type of waves that are confined at the boundary between two different media. They are used in subwavelength microscopy, molecular chemistry, cancer research, etc. [1][2][3] The most popular kind of surface wave is a plasmon formed at the boundary of metallic and dielectric media. Recently, the possibility to organize lossless surface waves at the interface between two dielectric media has been discussed. Artigas and Torner 4 have proposed to use specially designed two-dimensional photonic crystals for observation of the so-called Dyakonov modes 5 localized at the surface of the crystal. The photonic structure plays the role of a uniaxial dielectric medium having controllable refractive indices for ordinary and extraordinary light modes.In this Brief Report we propose another type of lossless interface modes, which we call optical Tamm states ͑OTSs͒ by analogy with well-known Tamm states for electrons at crystal boundaries. 6 Contrary to waveguided surface modes 7 and to Dyakonov modes, Tamm states remain localized for any value of the in-plane wave vector ͑including the zero wave-vector͒. OTSs are formed inside the "light cone" limited by a k = / c condition, where k is the wave vector of light and is its frequency. Contrary to electronic Tamm states, optical Tamm states cannot be formed at the surface, but only at the interface between two photonic structures having overlapping band gaps. We propose a simple planar multilayer structure allowing for observation of the OTSs. Ten years ago, one of us developed a theory of Tamm states for electrons confined at the boundary of two semiconductor superlattices. 8 Now we have extended the method of Ref. 8 to describe OTSs. We show that they can be formed at the interface between two periodic dielectric structures having different periods. The OTS lies in the optical stop bands of both parts of the structure. Its in-plane dispersion is parabolic with an effective mass of the order of 10 −5 of a free electron mass. The splitting between TE and TM polarized Tamm states increases quadratically with the in-plane wave vector. We propose realistic multilayer structures based on the porous silicon allowing for observation of the OTSs.Let us consider the interface between two periodical dielectric structures composed by the pairs of layers of thicknesses a r , b r and refractive indices n a , n b on...