This review provides a comprehensive overview of the crystal structure and properties of Heusler alloys, focusing on Ni-Mn-based alloys. Heusler alloys, discovered in the early 20th century, are known for their unique functional properties, including shape memory effect, super elasticity, magnetic transformation, and magneto-caloric effect. The review discusses the structural crystallinity, order-disorder atoms, phase changes, and magnetic ordering in these alloys. The crystal structure of Heusler alloys is highly dependent on the specific elements involved, with different structures such as L21, B2, and martensite (L10, 10M, 14M) being observed. The review emphasizes the importance of phase transitions, including martensitic and magnetic transformations, and how they affect the structural and magnetic properties of the alloys. It also discusses the ability of Heusler compounds to regulate valence electron concentration through element substitution. The review highlights the impact of doping with elements like Cu and Pd on the structural and magnetic properties of Heusler alloys. The review also covers the classification of Heusler alloys into full-Heusler, half-Heusler, and inverse-Heusler alloys, as well as their crystallographic structures and properties. The review discusses the thermal properties of Heusler alloys, including their phase transitions and the role of the e/a ratio in determining the martensitic start temperature. The review concludes with the importance of understanding the structural and magnetic properties of Heusler alloys for their application in various technologies, including spintronics, thermoelectrics, and superconductors.This review provides a comprehensive overview of the crystal structure and properties of Heusler alloys, focusing on Ni-Mn-based alloys. Heusler alloys, discovered in the early 20th century, are known for their unique functional properties, including shape memory effect, super elasticity, magnetic transformation, and magneto-caloric effect. The review discusses the structural crystallinity, order-disorder atoms, phase changes, and magnetic ordering in these alloys. The crystal structure of Heusler alloys is highly dependent on the specific elements involved, with different structures such as L21, B2, and martensite (L10, 10M, 14M) being observed. The review emphasizes the importance of phase transitions, including martensitic and magnetic transformations, and how they affect the structural and magnetic properties of the alloys. It also discusses the ability of Heusler compounds to regulate valence electron concentration through element substitution. The review highlights the impact of doping with elements like Cu and Pd on the structural and magnetic properties of Heusler alloys. The review also covers the classification of Heusler alloys into full-Heusler, half-Heusler, and inverse-Heusler alloys, as well as their crystallographic structures and properties. The review discusses the thermal properties of Heusler alloys, including their phase transitions and the role of the e/a ratio in determining the martensitic start temperature. The review concludes with the importance of understanding the structural and magnetic properties of Heusler alloys for their application in various technologies, including spintronics, thermoelectrics, and superconductors.