Mammals have an XX/XY sex chromosomal sex determination system in which males represent the heterogametic sex. The Y-linked gene, SRY, determines sex by inducing the undifferentiated, bipotential gonads to differentiate as testes, which produce androgens and promote in this way the development of a male phenotype. Thus, in mammals, sex determination can be equated to testis determination, which involves several important cell processes, including Sertoli cell differentiation, mesonephric cell migration, testis cord formation, testis-specific vascularization, and myoid and Leydig cell differentiation. Many genes are currently known to be involved in testis development. Some of them, including SF1, WT1, GATA4 and FOG2, are necessary for the formation of the bipotential, undifferentiated gonad but also have important roles in testis differentiation. Others can be considered testis-promoting, differentaition and/or maintenance genes: these include SRY, SOX9, FGF9, PTGDS, SOX8, SOX3, NR0B1, PDGFRa, DMRT1, AMH, NGF, NTF3 and NGFR as the most important examples. Finally, there is a smaller group of genes which are involved in ovarian development and which can cause aberrant testis development if mutated, including RSPO1, WNT4, CTNNB1, FST, BMP2 and FOXL2. In this paper, we review our current knowledge on the function, spatio-temporal expression pattern and mutant sexual phenotypes associated with these genes, and discuss the various roles they play in gonad development.