This review article provides an overview of the current status and recent research advancements in amorphous oxide semiconductors (AOSs) and their thin-film transistors (TFTs). AOSs, particularly amorphous In–Ga–Zn–O (a-IGZO), are expected to be the channel material of choice for next-generation flat-panel displays due to their high mobility, large size compatibility, fast response times, and superior stability compared to conventional silicon and organic TFTs. The key points covered in the review include: 1. **Device Issues**: Most device issues such as uniformity, long-term stability against bias stress, and TFT performance have been resolved for a-IGZO TFTs. 2. **Commercial Progress**: Sixth-generation (6G) processes have been demonstrated for 32" and 37" displays, and an 8G sputtering apparatus and target have been developed. 3. **Stability and Instability**: The effect of deep subgap states on illumination instability has been revealed, and mechanisms for illumination instability under negative bias have been studied. 4. **Degradation Mechanisms**: Degradation mechanisms are classified into back-channel effects, interface traps, and donor states in annealed a-IGZO TFTs, with bulk defects also considered. 5. **Passivation Layers**: Dense passivation layers improve stability and photoresponse, making them necessary for practical applications. 6. **Electronic Structures**: Sufficient knowledge of electronic structures and electron transport in a-IGZO has been accumulated to construct device simulation models. The article also discusses the advantages of AOSs over conventional materials, such as compatibility with large glass substrates, low-temperature fabrication, and excellent uniformity. It highlights the progress in flexible, transparent, and system-on-glass/SOP displays, as well as the development of large-area fabrication techniques for mass production. The review concludes with a detailed analysis of the operation characteristics of a-IGZO TFTs, including mobility, subthreshold voltage swing, and comparison with a-Si:H TFTs.This review article provides an overview of the current status and recent research advancements in amorphous oxide semiconductors (AOSs) and their thin-film transistors (TFTs). AOSs, particularly amorphous In–Ga–Zn–O (a-IGZO), are expected to be the channel material of choice for next-generation flat-panel displays due to their high mobility, large size compatibility, fast response times, and superior stability compared to conventional silicon and organic TFTs. The key points covered in the review include: 1. **Device Issues**: Most device issues such as uniformity, long-term stability against bias stress, and TFT performance have been resolved for a-IGZO TFTs. 2. **Commercial Progress**: Sixth-generation (6G) processes have been demonstrated for 32" and 37" displays, and an 8G sputtering apparatus and target have been developed. 3. **Stability and Instability**: The effect of deep subgap states on illumination instability has been revealed, and mechanisms for illumination instability under negative bias have been studied. 4. **Degradation Mechanisms**: Degradation mechanisms are classified into back-channel effects, interface traps, and donor states in annealed a-IGZO TFTs, with bulk defects also considered. 5. **Passivation Layers**: Dense passivation layers improve stability and photoresponse, making them necessary for practical applications. 6. **Electronic Structures**: Sufficient knowledge of electronic structures and electron transport in a-IGZO has been accumulated to construct device simulation models. The article also discusses the advantages of AOSs over conventional materials, such as compatibility with large glass substrates, low-temperature fabrication, and excellent uniformity. It highlights the progress in flexible, transparent, and system-on-glass/SOP displays, as well as the development of large-area fabrication techniques for mass production. The review concludes with a detailed analysis of the operation characteristics of a-IGZO TFTs, including mobility, subthreshold voltage swing, and comparison with a-Si:H TFTs.