This best practice guide provides information about IBM Power architecture machines in order to enable users of these systems to achieve good performance of their applications. The guide covers a wide range of topics, from the detailed description of the hardware, through information about the basic production environment, to information about porting and submitting jobs and tools and strategies for analysing and improving the performance of applications. Since this guide deals with a class of machines rather than with a single Tier-0 system, there is some room for variation. In fact, particularly at the levels of node interconnect technology and topology and the I/O subsystem there is considerable freedom of choice in design, and quite a few trade offs can be made in various implementation details. Hence the networks and node interconnects, and I/O subsystems of existing installations are quite system specific - rather than architecture specific. Consequently these are described in this guide at a system specific level of detail.