When women have fewer opportunities than men to develop their capacities in an egalitarian way, then we say that there is a gender gap. Although great conceptual and methodological advances have been achieved, and national policies have been developed in some countries to put an end to gender inequalities, what has really happened, in general, is an attenuation of those inequalities. This book reviews research in the study of the gender gap and have included herein a statistical analysis on gender and ethnicity; gender differences in second language learning; the gender pay gap in the self-employed in the U.S., evolutionary aspects of sex differences and gender disparities between men and women scientists in academia.