ARP/wARP is a software suite designed for automated macromolecular model building in X-ray crystallography. It provides tools for obtaining a macromolecular model automatically, including iterative protein model building, secondary structure construction, flexible loop building, ligand placement, and ordered water molecule detection. The software is user-friendly, accessible through a graphical user interface or command line, and suitable for non-expert users. ARP/wARP has been widely used in structural genomics and the study of complex macromolecular assemblies and membrane proteins, contributing to thousands of structure determination experiments. The software is available for free and can be downloaded from the ARP/wARP website. It is compatible with various Linux platforms and can run on Intel Itanium2 and Power PC machines. ARP/wARP's performance is generally good at higher crystallographic resolutions, but it can still produce useful models at lower resolutions. The software includes detailed instructions for data preparation, model building, and troubleshooting, ensuring efficient and accurate model building for researchers.ARP/wARP is a software suite designed for automated macromolecular model building in X-ray crystallography. It provides tools for obtaining a macromolecular model automatically, including iterative protein model building, secondary structure construction, flexible loop building, ligand placement, and ordered water molecule detection. The software is user-friendly, accessible through a graphical user interface or command line, and suitable for non-expert users. ARP/wARP has been widely used in structural genomics and the study of complex macromolecular assemblies and membrane proteins, contributing to thousands of structure determination experiments. The software is available for free and can be downloaded from the ARP/wARP website. It is compatible with various Linux platforms and can run on Intel Itanium2 and Power PC machines. ARP/wARP's performance is generally good at higher crystallographic resolutions, but it can still produce useful models at lower resolutions. The software includes detailed instructions for data preparation, model building, and troubleshooting, ensuring efficient and accurate model building for researchers.