The paper reviews the theoretical underpinnings, techniques, and results of estimating the CO-to-H₂ conversion factor (XCO) in different environments. In the Milky Way disk, a recommended XCO value of 2 × 10²⁰ cm⁻²(K km s⁻¹)⁻¹ with a ±30% uncertainty is provided. Studies of other "normal galaxies" yield similar values but with greater scatter and systematic uncertainty. XCO increases with decreasing metallicity, dropping sharply below a metallicity of approximately 1/3–1/2 solar, consistent with model predictions that identify shielding as a key parameter. XCO appears to decrease in the central bright regions of some galaxies, often coinciding with regions of bright CO emission and high stellar surface density. This lower XCO is also observed in starburst galaxies, where several lines of evidence point to a lower CO-to-H₂ conversion factor. At high redshift, direct evidence for the conversion factor remains scarce, but dynamical modeling and other arguments are reviewed. The paper discusses the historical perspective, CO excitation, theoretical basis, and insights from cloud models, emphasizing the importance of understanding the structure and dynamics of molecular clouds in setting XCO values.The paper reviews the theoretical underpinnings, techniques, and results of estimating the CO-to-H₂ conversion factor (XCO) in different environments. In the Milky Way disk, a recommended XCO value of 2 × 10²⁰ cm⁻²(K km s⁻¹)⁻¹ with a ±30% uncertainty is provided. Studies of other "normal galaxies" yield similar values but with greater scatter and systematic uncertainty. XCO increases with decreasing metallicity, dropping sharply below a metallicity of approximately 1/3–1/2 solar, consistent with model predictions that identify shielding as a key parameter. XCO appears to decrease in the central bright regions of some galaxies, often coinciding with regions of bright CO emission and high stellar surface density. This lower XCO is also observed in starburst galaxies, where several lines of evidence point to a lower CO-to-H₂ conversion factor. At high redshift, direct evidence for the conversion factor remains scarce, but dynamical modeling and other arguments are reviewed. The paper discusses the historical perspective, CO excitation, theoretical basis, and insights from cloud models, emphasizing the importance of understanding the structure and dynamics of molecular clouds in setting XCO values.