Specifically, we explore three key usage modes (see Figure 1): • HPC in the Cloud , in which researchers out - source entire applications to current public and/ or private Cloud platforms; • HPC plus Cloud , focused on exploring scenarios in which clouds can complement HPC/grid re - sources with cloud services to support science and engineering application workflows—for ex - ample, to support heterogeneous requirements or unexpected spikes in demand; and • HPC as a Service , focused on exposing HPC/grid resources using elastic on-demand cloud abstrac - tions, aiming to combine the flexibility of cloud models with the performance of HPC systems