MacDiff is a program for analysis and display of X-ray powder diffraction patterns on Apple Macintosh platforms. It is designed as a complement to the XRD applications distributed by Philips or Siemens and it is equipped with a GUI to facilitate user-friendly and interactive processing of XRD profiles. It offers conventional XRD profile-correction processes (e.g. outliers, smoothing, divergence, alpha2-stripping) and supports qualitative and quantitative phase analysis. Moreover, by its programmable peak analysis (recording of intensities, peak areas, half widths, etc.) it offers an automated analysis of entire series of diffractogrammes. By means of peak fits the contributions of coinciding lines can be calculated. The user has at his disposal the capability of continuous measurement checks and the possibility to perform corrections as well (e.g. by manual or computed background adaptations). For peak indication the user has the option to employ variable peak data sets. High resolution hardcopies of the diffractogrammes can be produced. The standard single-scan formats (Philips-ADP or some Siemens-RAW, ASCII, and others) are identified automatically. Data and plots can be exported to most of the available table-processing or vector-graphics programs. The applications of MacDiff range from routine analysis in sedimentology, especially clay minerals, over analysis of X-ray diffractogrammes of all types of rocks.