RADICAL-EnTK (Ensemble Toolkit) is a Python framework for developing and executing applications comprised of multiple sets of tasks, aka ensembles. Ensemble Toolkit has the following unique features, (i) abstractions that enable the expression of various task graphs, (ii) abstraction of resource management and task execution, (iii) Fault tolerance as a first order concern and (iv) well-established runtime capabilities to enable efficient and dynamic usage of grid resources and supercomputers.