Over the last twenty years, developed countries have been characterised by a relative increase in the demand for skilled workers compared with unskilled, showing a "skill bias" tendency. Not all countries and sectors have been affected in the same manner. This paper summarises main theoretical models and empirical evidence explaining this upskilling. The main explanation is attributed to the non-neutrality of technological change ("Skill Bias Technological Change") which turns out to be complementary to skilled workers and substitute to unskilled ones especially in US and UK. Less univocal results supporting the SBTC in the European countries have lead to alternative/complementary explanations. The second strand of the literature is based on the hypothesis that the increasing diffusion of new organizational practices, within more flexible firms, plays an important role in the growing demand for skilled workers. Moreover it seems there is a super-addictive effect of new technologies and new human resource practices on the skill demand. Finally, the last potential source of the skill bias - focused on the international dimension of production - explains the labour demand trend through the international division of work between developed and developing countries - this explanation seems to be empirically less robust than the previous two. In the conclusions some educational and training policies are suggested in order to reduce possible skill-mismatches on the labour market.