A review is made of the principal properties of mercury telluride which is a member of a new class of substances--zero-gap semiconductors. The factors responsible for the zero-gap state of mercury chalcogenides are discussed. It is shown that an inverted band structure is formed mainly because of relativistic corrections. Specific properties of mercury telluride are related to its zero gap, $p$-type electron states in the conduction band, nonparabolicity of this band, resonance impurity states, and permittivity anomalies. An analysis is made of the conditions for the appearance of a forbidden gap in mercury telluride under the influence of external factors.