Topological crystalline insulators (TCIs) are a new class of materials where the topological properties arise from crystal symmetries. This study predicts that SnTe, a semiconductor, is a TCI with mirror symmetry. The key feature is the presence of metallic surface states with an even number of Dirac cones on high-symmetry surfaces like {001}, {110}, and {111}. These surface states are protected by the reflection symmetry of the crystal with respect to the {110} mirror plane and form a robust, high-mobility chiral electron gas. The surface states can be tuned by breaking the mirror symmetry via elastic strain or an in-plane magnetic field, leading to a continuously tunable band gap. This makes SnTe promising for applications in thermoelectrics, infrared detection, and tunable electronics. Related semiconductors like PbTe and PbSe can also become TCIs through band inversion via pressure, strain, or alloying. The study shows that SnTe has a nonzero mirror Chern number, indicating a topological phase transition. First-principles calculations confirm that SnTe has an intrinsically inverted band structure, with the conduction band edge derived from Te and the valence band edge from Sn. This inversion leads to a topological phase transition, with SnTe at ambient pressure being a TCI with a mirror Chern number of 2. The surface states of SnTe are robust against disorder and protected by crystal symmetries. They exhibit anisotropic Dirac points and can be detected via ARPES and tunneling spectroscopy. The surface states also show a Lifshitz transition, where the Fermi surface topology changes with Fermi energy. The study also explores how surface states can be manipulated by structural distortions or magnetic fields, leading to different gapped phases. These phases are protected by crystal symmetries and can form domain walls with unique electronic properties. The results highlight the potential of SnTe and related materials for future topological devices.Topological crystalline insulators (TCIs) are a new class of materials where the topological properties arise from crystal symmetries. This study predicts that SnTe, a semiconductor, is a TCI with mirror symmetry. The key feature is the presence of metallic surface states with an even number of Dirac cones on high-symmetry surfaces like {001}, {110}, and {111}. These surface states are protected by the reflection symmetry of the crystal with respect to the {110} mirror plane and form a robust, high-mobility chiral electron gas. The surface states can be tuned by breaking the mirror symmetry via elastic strain or an in-plane magnetic field, leading to a continuously tunable band gap. This makes SnTe promising for applications in thermoelectrics, infrared detection, and tunable electronics. Related semiconductors like PbTe and PbSe can also become TCIs through band inversion via pressure, strain, or alloying. The study shows that SnTe has a nonzero mirror Chern number, indicating a topological phase transition. First-principles calculations confirm that SnTe has an intrinsically inverted band structure, with the conduction band edge derived from Te and the valence band edge from Sn. This inversion leads to a topological phase transition, with SnTe at ambient pressure being a TCI with a mirror Chern number of 2. The surface states of SnTe are robust against disorder and protected by crystal symmetries. They exhibit anisotropic Dirac points and can be detected via ARPES and tunneling spectroscopy. The surface states also show a Lifshitz transition, where the Fermi surface topology changes with Fermi energy. The study also explores how surface states can be manipulated by structural distortions or magnetic fields, leading to different gapped phases. These phases are protected by crystal symmetries and can form domain walls with unique electronic properties. The results highlight the potential of SnTe and related materials for future topological devices.