Ten years of R&D on Hall plasma thrusters (HPTs) in former USSR led to successful tests in space as early as 1972. Nevertheless, they remain actively investigated within the perspective of a routine use in space technologies. This long story underlines the contrast between a rather simple concept and a complex physical background, up to now not fully understood in a quantitative way. This lack of knowledge limits the development of fully predictive simulation codes. Industrial developments are, nevertheless, under way, with new goals in terms of thruster performances. Illustrations of such developments are given, but this paper is mainly focused on a review of recent results and open questions underlined through a coordinated research programme.