This Recommendation is reporting the elaboration of Testbed as a Service APIs based on the requirements and reference model with properties of relevance for delivering Testbed as a Service (TaaS), to complement and extend the Recommendation ITU-T Q.4068. It is more particularly focused on the user interface, services, and requirements to address end-user needs when remotely accessing testbeds through APIs in order to deliver adequate user experience. From this point, the Recommendation elaborates the related terms and definitions, requirements, reference model with properties of relevance for TaaS, and interoperability requirements for virtualizing and delivering modular and scalable TaaS on top of existing and future testbed infrastructures, including federated ones. The TaaS is able to list the assets provided by the different testbeds and expose them through dedicated APIs based on the Recommendation ITU-T Q.4068.