The authors present the first monograph on electronic properties, lattice dynamics and phase transitions in TlX and TlMX2 (M = Ga, In; X = Se, S, Te) compounds. These chalcogenides belong to a family of the low-dimensional ?3?6 and A3B3C62 semiconductors possessing chain or layered structure. They are of significant interest because of highly anisotropic properties, semiconductor and photoconductor behaviour, non-linear effects in their current-voltage characteristics (including a region of negative differential resistance), switching and memory effects, second harmonic optical generation, ferroelectricity, relaxor behaviour and potential applications in optoelectronic devices. We review (i) crystal structure of the compounds, (ii) their transport properties under ambient conditions, (iii) experimental studies of the electronic structure, (iv) band structure calculations, (v) transport properties, semiconductor-metal phase transitions and band structure under high pressure, (vi) ferroelectric/structural phase transitions at ambient pressure, soft modes and incommensurate states, (vii) electronic nature of the ferroelectric phase transitions, (viii) lattice dynamics in ?3?6 and A3B3C62 crystals and their solid solutions, (ix) elastic properties and (x) recently discovered relaxor behaviour, nanodomains and quantum dots in doped and irradiated crystals. Physical properties of some related low-dimensional compounds are also discussed. New perspectives in studying and application of the aforementioned semiconductors are considered. The monograph is addressed to physicists, materials scientists, engineers and students who are interested in physics of the low-dimensional compounds, lattice dynamics, phase transitions and ferroelectricity.