During the nineties, Brazil experienced a remarkable increase in its unemployment rate. The rise in unemployment was more intense for unskilled and semi-skilled workers relative to skilled workers. The objective of this thesis is to identify the determinants for the different patterns of unemployment rate by skill during the nineties. The thesis is organized into six chapters, including the introduction and the conclusion. The chapters 2 and 3 of the thesis show that occurred an increase of skilled workers productivity relative to unskilled and semi-skilled during the nineties, which is consistent with the technological progress that followed the trade liberalization. As a consequence of these changes, the skilled workers unemployment rate decreased relative to the other groups. The chapter 4 presents evidences that the skilled relative unemployment rate decreased for the younger skilled cohorts, while older skilled cohorts experienced increases in its relative unemployment rate. The empirical evidence, obtained decomposing the unemployment rate by skill into age, period and cohort effects, shows that characteristics associated with younger skilled cohorts contributed for the reduction in this group unemployment rate. This could be explained by the fact that technological changes in the nineties were biased toward these workers because of their type and quality of education. The chapter 5 shows that increases in the wage pressure were important to the relative unemployment rate pattern. During the nineties the household retirement income per capita rose sharply in Brazil, and it happened in a more intense way for the unskilled workers than for the other groups. According to the argument of this chapter, the household retirement income increasing affected workers reservation wage, which augmented the wage pressure. Consequently, the higher wage pressure implied in larger unemployment rate. The empirical evidence shows that the household retirement income is positively associated with the unemployment rate for unskilled workers.