Separation assurance is a fundamental requirement for safe operations of air traffic. New paradigms of separation assurance will be required to accommodate the anticipated increase of air traffic in the future. This paper defines a taxonomy for the allocation of separation assurance functions along the air/ground and human/automation axes, and then builds a knowledge base from a comprehensive survey of separation assurance studies conduced over the past 12 years with an emphasis on high-fidelity human-in-the-loop simulations and operational evaluations. The goal of this effort is to identify trends and gaps in the current knowledge base of functional allocation for separation assurance, so that it may serve as a guide for planning future work. One finding is that limited delegation for arrival merging/spacing has been developed to a relatively high level of maturity. Another finding is that various aspects of automated concepts for ground-based and airborne separation assurance have been well studied; the key challenge going forward is system level integration and evaluation.