The article discusses the scaling and assessment of diffraction data in crystallography. It outlines the physical factors affecting measured diffraction intensities and the scaling models used to normalize the data. After scaling, the intensities can be analyzed to determine the real resolution, detect bad regions, assess radiation damage, and evaluate data quality. The significance of anomalous signals is assessed using probability and correlation analysis. The CCP4 program SCALA is described, along with the new program POINTLESS for determining Laue groups. The article explains the physical reasons for differences in scale, including factors related to the incident X-ray beam, the crystal and diffracted beam, and the detector. It describes how these factors are modeled to correct measured intensities. The scaling model is chosen to reflect the diffraction experiment, and various models are used to account for changes in intensity over time and rotation. The correction factors are optimized to ensure internal consistency of the data by minimizing discrepancies between symmetry-related observations. The function minimized includes terms for the weighted average intensity and parameter constraints. The article also discusses the assessment of data quality, including real resolution, detection of bad data, and the significance of anomalous signals. It describes methods for outlier rejection and the analysis of multiple-wavelength data sets to detect anomalous signals. The determination of the Laue group, point group, and space group is discussed, with a focus on the new program POINTLESS. The article outlines the stages of space-group determination, including lattice symmetry, Laue group symmetry, point-group symmetry, and space-group symmetry. Scoring functions are used to determine the Laue group, considering the agreement between observations related by potential symmetry. The article also covers the future directions for POINTLESS, including the assessment of intensity statistics and systematic absences to score possible space groups and detect twinning. The algorithms used in SCALA are described, including the scaling model, error estimation, and outlier rejection. The article concludes with references to various studies and authors contributing to the field of crystallography.The article discusses the scaling and assessment of diffraction data in crystallography. It outlines the physical factors affecting measured diffraction intensities and the scaling models used to normalize the data. After scaling, the intensities can be analyzed to determine the real resolution, detect bad regions, assess radiation damage, and evaluate data quality. The significance of anomalous signals is assessed using probability and correlation analysis. The CCP4 program SCALA is described, along with the new program POINTLESS for determining Laue groups. The article explains the physical reasons for differences in scale, including factors related to the incident X-ray beam, the crystal and diffracted beam, and the detector. It describes how these factors are modeled to correct measured intensities. The scaling model is chosen to reflect the diffraction experiment, and various models are used to account for changes in intensity over time and rotation. The correction factors are optimized to ensure internal consistency of the data by minimizing discrepancies between symmetry-related observations. The function minimized includes terms for the weighted average intensity and parameter constraints. The article also discusses the assessment of data quality, including real resolution, detection of bad data, and the significance of anomalous signals. It describes methods for outlier rejection and the analysis of multiple-wavelength data sets to detect anomalous signals. The determination of the Laue group, point group, and space group is discussed, with a focus on the new program POINTLESS. The article outlines the stages of space-group determination, including lattice symmetry, Laue group symmetry, point-group symmetry, and space-group symmetry. Scoring functions are used to determine the Laue group, considering the agreement between observations related by potential symmetry. The article also covers the future directions for POINTLESS, including the assessment of intensity statistics and systematic absences to score possible space groups and detect twinning. The algorithms used in SCALA are described, including the scaling model, error estimation, and outlier rejection. The article concludes with references to various studies and authors contributing to the field of crystallography.