Gctf is a GPU-accelerated program for accurate and real-time determination and correction of the contrast transfer function (CTF) in cryo-electron microscopy (cryoEM). It improves upon existing methods by using a fast '1D search plus 2D refinement (1S2R)' procedure, enhancing both speed and accuracy. The program includes novel diagnostic methods such as equiphase averaging (EFA) and self-consistency verification, which are crucial for near-atomic resolution reconstructions. Gctf is independent and can be integrated with other cryoEM software like Relion and Frealign. It processes thousands of micrographs quickly using a single GPU card and provides accurate local defocus refinement for each particle and movie frame. The program also includes features for automatic self-consistency verification and micrograph quality evaluation, facilitating automation in cryoEM. Gctf has been tested on various datasets, demonstrating its effectiveness in improving CTF parameters and enabling high-resolution 3D reconstructions. The program's speed and accuracy are further enhanced by GPU acceleration, optimized programming strategies, and the 1S2R procedure. It can handle challenging cases, including large astigmatism and defocus variations, through methods like resolution extension (RE) and B-factor switching (BS). Gctf's ability to accurately determine CTF for individual frames and its robustness in handling different input parameters make it a powerful tool for cryoEM data processing. The program has been successfully applied to various datasets, including those with high-resolution structures, and has shown significant improvements in defocus accuracy and overall reconstruction quality.Gctf is a GPU-accelerated program for accurate and real-time determination and correction of the contrast transfer function (CTF) in cryo-electron microscopy (cryoEM). It improves upon existing methods by using a fast '1D search plus 2D refinement (1S2R)' procedure, enhancing both speed and accuracy. The program includes novel diagnostic methods such as equiphase averaging (EFA) and self-consistency verification, which are crucial for near-atomic resolution reconstructions. Gctf is independent and can be integrated with other cryoEM software like Relion and Frealign. It processes thousands of micrographs quickly using a single GPU card and provides accurate local defocus refinement for each particle and movie frame. The program also includes features for automatic self-consistency verification and micrograph quality evaluation, facilitating automation in cryoEM. Gctf has been tested on various datasets, demonstrating its effectiveness in improving CTF parameters and enabling high-resolution 3D reconstructions. The program's speed and accuracy are further enhanced by GPU acceleration, optimized programming strategies, and the 1S2R procedure. It can handle challenging cases, including large astigmatism and defocus variations, through methods like resolution extension (RE) and B-factor switching (BS). Gctf's ability to accurately determine CTF for individual frames and its robustness in handling different input parameters make it a powerful tool for cryoEM data processing. The program has been successfully applied to various datasets, including those with high-resolution structures, and has shown significant improvements in defocus accuracy and overall reconstruction quality.