This paper considers ancient and some modern thought about the general characteristics of ‘client kings.’ Arguably exceptional cases (especially Parthians) and key issues (especially succession) are examined in the larger framework of Roman imperialist ideology as well as, where possible, from kings’ viewpoints. Differences and changes are seen as variations on an elastic, but integral theme. That elasticity, enhanced by the language of courtesy, obstructs narrow modern definition, but crucially explains the success of this Roman imperial strategy. Tacitus is central to all this. Finally, a new reading of Annals 2. 61 arises from this and closer consideration of Roman notions of the Red Sea region (“Ethiopia”, India etc.), so that the passage becomes inconclusive to discussions of the completion-date of the Annals, on which it has oten been seen as important.