The IBM POWER8 (TM) architecture introduces many novel features that improve the overall performance and energy management of systems based on this new platform. To cover a wide range of workloads, the design team focused on not just the traditional server workloads like transaction processing and enterprise resource planning, but also on emerging workloads such as big data, analytics, and cloud. Designed for openness, the POWER8 platform enables other technology providers to innovate freely with OpenPower (TM). In designing the most critical POWER8 processor features, the development team relied on state-of-the-art performance and energy models for design optimization and trade-off studies, and for projecting the system-level performance for the most important workloads. In this paper, we describe these models in detail and spotlight how the models were used to influence the design of specific POWER8 features.