TerraSwarm applications, or swarmlets, are characterized by their ability to dynamically recruit resources such as sensors, communication networks, computation, and information from the cloud; to aggregate and use that information to make or aid decisions; and then to dynamically recruit actuation resources. The TerraSwarm vision cannot be achieved by a single vendor providing the components as an integrated system. What is needed instead is the swarm equivalent of the common, general framework that has recently enabled smartphones and similar devices to rapidly deploy and serve a vast range of often unanticipated applications by recruiting resources and composing services. The SwarmOS must support continual reconfiguration of applications and of its own service definitions without ever having the luxury of a clean restart. It must also support richly heterogeneous computing. TerraSwarm protocols will need to detect compromises, distinguish trusted from untrusted data and resources, and be robust to the presence of a certain number of malicious nodes.