Work Package 7 ‘Enabling Petascale Applications: Efficient Use of Tier-0 Systems’ (WP7) ensures the effective exploitation of the PRACE Tier-0 systems by increasing scalability and performance of applications. While the focus in WP7 is primarily on enabling applications for Tier-0 systems, the tasks should also benefit applications performance on Tier-1 systems. Most of the activities had to commence on existing Tier-1 systems while waiting for the installation of the Tier-0 systems. Task 7.3 is called ‘Efficient Use of PRACE Systems’. Efficient use of the PRACE Tier-0 systems requires detailed knowledge of architecture-specific factors influencing performance, including compilers, tools and libraries. The main goal of this task is to investigate such issues, collect best practices on how to achieve good performance on the systems, and disseminate this knowledge to users. Topics for best practice guides include: optimal porting of applications (e.g. choice of numerical libraries and compiler options); architecture-specific optimisation and petascaling techniques; optimal system environment (e.g. tuneable system parameters, job placement and optimised system libraries); debuggers, performance analysis tools and programming environment. Task 7.3 covers two Tier-0 systems (JUGENE and Curie) and two common Tier-1 system families (Cray XE and IBM Power) with the potential to evolve in a Tier-0 system. Finally, since the four best practice guides comprise some 50 – 80 pages each, we decided not to include them as separate chapters in this report but to refer to the online versions on the PRACE RI web site instead.