This chapter describes the physical infrastructure of cloud data centers and the facilities in them. It begins by defining the organization of racks into aisles and pods, then proceeds to examine the topics of power and cooling. Although it may seem irrelevant to deployment of software systems in a cloud, power considerations figure into many discussions throughout the book. The chapter goes on to describe data center networks, including network switches used to interconnect servers. It explains and contrasts the north-south traffic that arises from web server access and east-west traffic that arises from modern uses of cloud facilities. The chapter explains how early data center networks used a hierarchical architecture, and how modern data center networks use a leaf-spine architecture that employs a folded-Clos design to permit arbitrary scaling without a bottleneck. The chapter explains that data centers keep storage separate from servers and describes how storage access now uses the same leaf-spine network as other traffic (i.e., how all data center networking has been unified to use the Internet Protocol).