Hobbes was a Sandia-led collaboration between four national laboratories and eight universities supported by the DOE Office of Science Advanced Scientific Computing Research program office. The goal of this three-year project was to deliver an operating system for future extreme-scale parallel computing platforms that will address the major technical challenges of energy efficiency, managing massive parallelism and deep memory hierarchies, and providing resilience in the presence of increasing failures. Our approach was to enable application composition through lightweight virtualization. Application composition is a critical capability that will be the foundation of the way extreme-scale systems must be used in the future. The tighter integration of modeling and simulation capability with analysis and the increasing complexity of application workflows demand more sophisticated machine usage models and new system-level services. Ensemble calculations for uncertainty quantification, large graph analytics, multi-materials and multi-physics applications are just a few examples that are driving the need for these new system software interfaces and mechanisms for managing memory, network, and computational resources. Rather than providing a single unified operating system and runtime system that supports several parallel programming models, Hobbes leveraged lightweight virtualization to provide the flexibility to construct and efficiently execute custom OS/R environments. Hobbes extended previous work on the Kitten lightweight operating system and the Palacios lightweight virtual machine monitor.