Attosecond transient absorption spectroscopy (ATAS) is an all-optical pump-probe technique that employs attosecond pulses (from the extreme ultraviolet to soft X-ray) to excite or probe a system, enabling real-time tracking of electronic transitions, quantum state evolution, and energy transfer processes. This approach offers key advantages: (1) ultrafast temporal resolution (sub-femtosecond) combined with high spectral resolution (millielectronvolt level); (2) broadband excitation of multiple quantum states, allowing simultaneous detection across multiple energy levels; and (3) element- and site-specific insights afforded by inner-shell to valence transition measurements that reveal charge transfer dynamics, spin state changes, and local structural evolution. To date, significant breakthroughs have been achieved in atomic/molecular physics, electronic coherent dynamics, and strong-field physics using ATAS. This paper systematically reviews the technical principles and theoretical models associated with ATAS employing moderately strong near-infrared pulses, analyzes recent progress in applications to both gas-phase and condensed-phase systems, and explores its future prospects in ultrafast physical chemistry and quantum materials. In gas-phase environments, ATAS has demonstrated significant capabilities in probing energy level shifts and population transfers in atomic systems, as well as capturing nonadiabatic dynamics and charge migration in diatomic and polyatomic molecules. In contrast, within condensed-phase systems, the technique has been effectively used to study the ultrafast dynamics of carriers in semiconductors and to examine the interaction dynamics of localized electrons in insulators and transition metals. Given the rapid evolution of attosecond laser technologies and the distinct advantages of the ATAS detection approach, the paper also outlines potential future directions. These prospects promise to further extend the frontiers of ultrafast spectroscopy and to drive advances across a range of disciplines in both fundamental research and technological applications.