Trade SIAs have been conceived to help policy makers to maximise trade-offs in the liberalisation process, assuming that positive trade-offs ultimately exist across sustainable development pillars. Existing Trade SIAs tend to anticipate objective impacts of trade liberalisation at global level. To that aim, they use indicators that are likely to give an image of the reality after liberalisation. The relevance of these indicators for sustainable development and the relevance of their aggregation rest on each society's views and priorities on sustainability. Indeed, the varied appreciations of trade liberalisation's impact values - meaning impact valued within each society, and not only physical impact value - explain much of WTO negotiators' misunderstanding in trade talks. Trade SIAs, by unveiling differences in the valuation of trade liberalisation impacts across countries, could ideally help trade negotiations progress in areas where convergence in sustainability meanings and values has been made clear. In the meantime, it should help isolate deadlocks stemming from apparently incompatible views on what sustainability means for each different country. By reviewing and comparing the challenges of trade liberalisation in 1999 and 2006 from a Trade SIA perspective, we argue that new directions for Trade SIAs seem needed, which we summarise in conclusion.