This chapter provides a comprehensive overview of the software ecosystem that underpins the operation of Warehouse-Scale Computers (WSCs). It covers the layered structure of the software stack, focusing on distributed systems infrastructure including foundational services and cluster-level infrastructure for resource management, unstructured and structured storage systems like Colossus and Spanner, and distributed networking extending beyond data centers. The discussion emphasizes the importance of infrastructure management, encompassing resource provisioning, configuration management, observability, and tracing tools. We also highlight the significance of practices like DevOps and Site Reliability Engineering (SRE) in managing WSCs effectively. Finally, we discuss server-level software, including the operating system, firmware, containerization, and virtualization, with a focus on security aspects like the Silicon Root of Trust.