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Helium, as the ash of burning D-T plasma, is an unavoidable impurity component necessarily present already in near future tokamak experiments with significant alpha particle heating. Its efficient removal from the burning zone of a D-T fusion reactor (or lack thereof) will play a key role in the path towards achievement of economic fusion power production. A survey is given of the issues related to this question. Since there is as yet no experimental experience with thermonuclear plasmas significantly heated by fusion products, this review is based on results from simulation experiments of helium injection into hydrogen or deuterium tokamak plasmas, and from numerical transport code work. Both kinds of results are discussed with reference to handy ignition criteria obtained for steady D-T burning, which have been reformulated in terms relevant for the ash removal problem.

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