The paper discusses the merits of airborne and ground-based concepts for implementing TBO and their impact on capacity, safety, efficiency, and sustainability. Progress made in 40 years is illustrated based on the comparison of two samples of traffic inbound to London-Heathrow. Airborne and ground-based automation concepts for facilitating TBO are exposed to the challenges observed. The advantages of Augmented Intelligence (AI) in ground-based, Advanced Interval Management (AIM) become evident.