The 21 cm transition of neutral hydrogen at high redshifts (z ~ 50) offers a new window into the early phases of cosmic structure formation. Observations of this transition can provide insights into the matter power spectrum, the formation of the cosmic web, the first luminous sources, and the reionization of the intergalactic medium (IGM). The epoch of reionization is particularly interesting because large HII regions will seed significant fluctuations in the 21 cm background. However, detecting this signal is challenging due to foregrounds from the Milky Way and extragalactic sources, especially at low frequencies. Despite these difficulties, the redshifted 21 cm line has the potential to offer unique insights into the high-redshift Universe, complementing other probes and providing the only direct, three-dimensional view of structure formation from z ~ 200 to z ~ 6. The review covers the fundamental physics of the 21 cm transition, the global evolution of the IGM, the power spectrum, and the prospects for observations at low frequencies. It also discusses complementary observations and concludes with a discussion of the challenges and potential rewards of 21 cm line cosmology.The 21 cm transition of neutral hydrogen at high redshifts (z ~ 50) offers a new window into the early phases of cosmic structure formation. Observations of this transition can provide insights into the matter power spectrum, the formation of the cosmic web, the first luminous sources, and the reionization of the intergalactic medium (IGM). The epoch of reionization is particularly interesting because large HII regions will seed significant fluctuations in the 21 cm background. However, detecting this signal is challenging due to foregrounds from the Milky Way and extragalactic sources, especially at low frequencies. Despite these difficulties, the redshifted 21 cm line has the potential to offer unique insights into the high-redshift Universe, complementing other probes and providing the only direct, three-dimensional view of structure formation from z ~ 200 to z ~ 6. The review covers the fundamental physics of the 21 cm transition, the global evolution of the IGM, the power spectrum, and the prospects for observations at low frequencies. It also discusses complementary observations and concludes with a discussion of the challenges and potential rewards of 21 cm line cosmology.