Preface: Jimenez, R.; How to Make a Gonad: Cellular Mechanisms Governing Formation of the Testes and Ovaries: Ungewitter, E.K.; Yao, H.H.-C.; Genetic Control of Testis Development: Sekido, R.; Lovell-Badge, R.; Genetic Control of Ovarian Development: Tevosian, S.G.; Precious Cargo: Regulation of Sex-Specific Germ Cell Development in Mice: Bowles, J.; Koopman, P.; Steroid Hormones and the Development of Reproductive Organs: Morohashi, K.; Baba, T.; Tanaka, M.; The Molecular Genetics of Ovarian Differentiation in the Avian Model: Ayers, K.L.; Sinclair, A.H.; Smith, C.A.; Environmental Sex Determination Mechanisms in Reptiles: Merchant-Larios, H.; Diaz-Hernandez, V.; Is a Sex-Determining Gene(s) Necessary for Sex-Determination in Amphibians? Steroid Hormones May Be the Key Factor: Nakamura, M.; Diversity and Plasticity of Sex Determination and Differentiation in Fishes: Kobayashi, Y.; Nagahama, Y.; Nakamura, M.; Gene Mutations Associated with Anomalies of Human Gonad Formation: Bashamboo, A.; McElreavey, K.; Natural Exceptions to Normal Gonad Development in Mammals: Jimenez, R.; Barrionuevo, F.J.; Burgos, M.; Evo-Devo of the Germline and Somatic Gonad in Nematodes: Pires-daSilva, A.; Parihar, M.