It is vital to examine the population composition, i.e. how the population being studied is structured. In demography, to understand the phenomena studied (firstly, births and deaths), it is common practice to examine the number of women in relation to the number of men and to distinguish the distribution of individuals by age. The most common representation of this gender and age composition is the “age pyramid”. It is important to know the age and gender composition, as this largely determines a population's demographic dynamics. Elements relating to gender and age composition will enable better understanding of the dynamics outlined in the following chapters. In this chapter therefore, we will present the composition of the Mediterranean population from the perspective of sex ratios on the one hand, and the age distribution of populations on the other. We will also consider the implications of differences between countries at the sub-national level.