Heusler alloys are theoretically predicted to become half-metals at room temperature (RT). The advantages of using these alloys are good lattice matching with major substrates, high Curie temperature above RT and intermetallic controllability for spin density of states at the Fermi energy level. The alloys are categorised into half- and full-Heusler alloys depending upon the crystalline structures, each being discussed both experimentally and theoretically in Section 2. Fundamental properties of ferromagnetic Heusler alloys are described in Section 3. Both structural and magnetic characterisations on an atomic scale are typically carried out in order to prove the half-metallicity at RT as described in Section 4. Atomic ordering in the Heusler-alloy films is directly observed by X-ray diffraction and is also indirectly probed via the temperature dependence of electrical resistivity. Element specific magnetic moments and spin polarisation of the Heusler alloy films are directly measured using X-ray magnetic circular dichroism and Andreev reflection, respectively. By employing these ferromagnetic alloy films in a spintronic device, efficient spin injection into a non-magnetic material and large magnetoresistance are discussed in Section 5. Fundamental properties of antiferromagnetic Heusler alloys are described in Section 6. Both structural and magnetic characterisations on an atomic scale are shown in Section 7. Atomic ordering in the Heusler-alloy films is indirectly measured by the temperature dependence of electrical resistivity. Antiferromagnetic configurations are directly imaged by X-ray magnetic linear dichroism and polarised neutron reflection. Section 8 explains applications of the antiferromagnetic Heusler-alloy films. The other non-magnetic Heusler alloys are listed in Section 9. A brief summary is provided at the end of this review.