The CO-to-H₂ conversion factor, X_CO, is crucial for estimating molecular gas mass in the interstellar medium. This review discusses the theoretical basis, techniques, and results of estimating X_CO in different environments. In the Milky Way, a recommended value is X_CO = 2 × 10²⁰ cm⁻² (K km s⁻¹)⁻¹ with ±30% uncertainty. Studies of other galaxies show similar values but with greater scatter. X_CO increases with decreasing metallicity, especially below 1/3-1/2 solar, consistent with shielding effects. In some galaxies, X_CO drops in central regions, coinciding with bright CO emission and high stellar density. In starburst galaxies, X_CO is lower due to different physical conditions. At high redshift, direct evidence is limited, but dynamical modeling and other arguments provide insights. CO is a key tracer of molecular gas because it is easily excited in cold molecular clouds. Its J=1→0 transition is optically thick, making it a useful tool for measuring molecular gas mass. The conversion factor, X_CO, relates observed CO intensity to molecular hydrogen column density. The mass-to-light ratio, α_CO, is derived from this relationship. X_CO and α_CO are both referred to as the CO-to-H₂ conversion factor. For X_CO = 2 × 10²⁰ cm⁻² (K km s⁻¹)⁻¹, α_CO is 4.3 M☉ (K km s⁻¹ pc²)⁻¹. This allows direct conversion of integrated flux density to molecular mass. Theoretical models suggest that X_CO depends on environmental factors such as gas density, temperature, and metallicity. In molecular clouds, X_CO varies with cloud properties, and in galaxies, it is influenced by the distribution of GMCs and their sizes. The virial mass of GMCs is related to their size and velocity dispersion, and the CO luminosity is proportional to the molecular mass. The "mist" model suggests that CO can trace molecular mass in galaxies due to the ensemble's optical thinness despite individual clouds being optically thick. The CO-to-H₂ conversion factor is also affected by the velocity dispersion of the gas. In regions with higher velocity dispersion, the luminosity increases, leading to an overestimation of molecular mass if X_CO is applied directly. Corrections for this effect are necessary, especially in galaxy centers where velocity dispersion is higher. The conversion factor also depends on the distribution of gas and total mass, with variations in GMC sizes and densities affecting X_CO. In optically thin conditions, X_CO is smaller, and the conversion factor is influenced by the abundance of CO and excitation temperature. Models of photodissociation regions (PDRs) show that X_CO varies with depth in the cloud and is affected by the radiation field and metallicity. Recent simulations and models suggest that X_CO depends on the densityThe CO-to-H₂ conversion factor, X_CO, is crucial for estimating molecular gas mass in the interstellar medium. This review discusses the theoretical basis, techniques, and results of estimating X_CO in different environments. In the Milky Way, a recommended value is X_CO = 2 × 10²⁰ cm⁻² (K km s⁻¹)⁻¹ with ±30% uncertainty. Studies of other galaxies show similar values but with greater scatter. X_CO increases with decreasing metallicity, especially below 1/3-1/2 solar, consistent with shielding effects. In some galaxies, X_CO drops in central regions, coinciding with bright CO emission and high stellar density. In starburst galaxies, X_CO is lower due to different physical conditions. At high redshift, direct evidence is limited, but dynamical modeling and other arguments provide insights. CO is a key tracer of molecular gas because it is easily excited in cold molecular clouds. Its J=1→0 transition is optically thick, making it a useful tool for measuring molecular gas mass. The conversion factor, X_CO, relates observed CO intensity to molecular hydrogen column density. The mass-to-light ratio, α_CO, is derived from this relationship. X_CO and α_CO are both referred to as the CO-to-H₂ conversion factor. For X_CO = 2 × 10²⁰ cm⁻² (K km s⁻¹)⁻¹, α_CO is 4.3 M☉ (K km s⁻¹ pc²)⁻¹. This allows direct conversion of integrated flux density to molecular mass. Theoretical models suggest that X_CO depends on environmental factors such as gas density, temperature, and metallicity. In molecular clouds, X_CO varies with cloud properties, and in galaxies, it is influenced by the distribution of GMCs and their sizes. The virial mass of GMCs is related to their size and velocity dispersion, and the CO luminosity is proportional to the molecular mass. The "mist" model suggests that CO can trace molecular mass in galaxies due to the ensemble's optical thinness despite individual clouds being optically thick. The CO-to-H₂ conversion factor is also affected by the velocity dispersion of the gas. In regions with higher velocity dispersion, the luminosity increases, leading to an overestimation of molecular mass if X_CO is applied directly. Corrections for this effect are necessary, especially in galaxy centers where velocity dispersion is higher. The conversion factor also depends on the distribution of gas and total mass, with variations in GMC sizes and densities affecting X_CO. In optically thin conditions, X_CO is smaller, and the conversion factor is influenced by the abundance of CO and excitation temperature. Models of photodissociation regions (PDRs) show that X_CO varies with depth in the cloud and is affected by the radiation field and metallicity. Recent simulations and models suggest that X_CO depends on the density