The Andromeda Galaxy (M31, NGC 224) is a barred spiral galaxy (type Sb) at 780 kpc (2.54 Mly), the nearest major galaxy to the Milky Way and the Local Group's most massive member. Its stellar mass is ~1.0-1.5×10¹² M☉; total mass including dark matter halo is ~1.2-2.0×10¹² M☉. The stellar disk spans ~220,000 ly in diameter. M31 hosts ~10¹² stars, with a prominent bulge containing a supermassive black hole (M31*) of mass ~1.4×10⁸ M☉. Its structure comprises a thin disk with spiral arms rich in young, metal-rich Population I stars and HII regions, a thick disk, and an extended halo of old, metal-poor Population II stars and ~460 globular clusters. Kinematic data indicate a radial velocity of -110 km/s toward the Milky Way; combined with proper motion measurements (Gaia, HST), ΛCDM simulations predict a merger in ~4.5 Gyr, forming a giant elliptical (Milkdromeda). M31 exhibits a star-forming ring at ~10 kpc, likely from a minor merger ~2 Gyr ago. Its interstellar medium contains ~10⁹ M☉ of neutral hydrogen (HI) and molecular gas, with a metallicity gradient declining radially. As the closest large spiral, M31 enables high-resolution studies of stellar populations, dynamics, and feedback processes critical for testing galaxy formation models within the ΛCDM framework.