The paper by H.J. Mo, Shude Mao, and Simon D.M. White explores the formation of galactic disks within hierarchical clustering models for structure formation. The authors assume that a rotationally supported disk with an exponential surface density profile forms with a mass and angular momentum that are fixed fractions of the surrounding dark halo's mass and angular momentum. They assume that haloes respond adiabatically to disk formation and that only stable disks can correspond to real systems. The predicted population of disks can match both present-day disks and the damped Lyα absorbers in QSO spectra, provided certain conditions are met: the disk mass is a few percent of the halo mass, the specific angular momentum of the disk is similar to that of the halo, and the present-day disks were assembled recently (at \( z \leq 1 \)). The observed scatter in the size-rotation velocity plane and the slope and scatter of the Tully-Fisher relation are reproduced. The zero-point of the Tully-Fisher relation is matched for a stellar mass-to-light ratio of 1 to 2 h in the I-band, consistent with observational values. High-redshift disks are predicted to be small and dense, potentially merging to form elliptical galaxies. The paper also discusses the implications for damped Lyα systems and the bias towards low surface brightness systems.The paper by H.J. Mo, Shude Mao, and Simon D.M. White explores the formation of galactic disks within hierarchical clustering models for structure formation. The authors assume that a rotationally supported disk with an exponential surface density profile forms with a mass and angular momentum that are fixed fractions of the surrounding dark halo's mass and angular momentum. They assume that haloes respond adiabatically to disk formation and that only stable disks can correspond to real systems. The predicted population of disks can match both present-day disks and the damped Lyα absorbers in QSO spectra, provided certain conditions are met: the disk mass is a few percent of the halo mass, the specific angular momentum of the disk is similar to that of the halo, and the present-day disks were assembled recently (at \( z \leq 1 \)). The observed scatter in the size-rotation velocity plane and the slope and scatter of the Tully-Fisher relation are reproduced. The zero-point of the Tully-Fisher relation is matched for a stellar mass-to-light ratio of 1 to 2 h in the I-band, consistent with observational values. High-redshift disks are predicted to be small and dense, potentially merging to form elliptical galaxies. The paper also discusses the implications for damped Lyα systems and the bias towards low surface brightness systems.