Basic facts about the summation of divergent power series are reviewed, both for series with non vanishing and for series with vanishing convergence radius. Particular attention is paid to the recent development that makes it possible, in the former case, to define summation in the whole Mittag-Lemer star and, in the latter case, to define summation when the point of expansion lies at the tip of a horn-shaped analyticity domain with zero opening angle. Relevance of these results to perturbative QCD is stressed in relation to current discussions concerning large-order estimates of perturbative QCD expansion coefficients. 665 667 668 668 669 670 612 674 614 676 678 679 679 68 1 682 684 685 685 686 681It is widely believed that perturbation series in quantum theory are mostly divergent. But the problem is not whether a power series is convergent or divergent, but rather whether