Breakthroughs and challenges of modern developmental biology and reproductive medicine.

In recent decades we have witnessed unprecedented progress in the field of the developmental biology of mammals. Building on 20 ^th century discoveries, we have managed to increase our understanding of the molecular and cellular mechanisms governing early mammalian embryogenesis and link them to other biological questions, such as stem cells, regeneration, cancer, or tissue and organ formation. Consequently, it has also led to a creation of a completely new branch of reproductive medicine, i.e. assisted reproductive technology (ART). In this Special Issue of The International Journal of Developmental Biology (Int. J. Dev. Biol.) we wished to review state-of-the-art research regarding early mammalian development, from fertilization up to the implantation stage, and discuss its potential meaning for practical applications, including ART. As an introduction to the issue we present a compilation of short essays written by the most renowned scientists in the field, working both in basic and clinical research. The essays are dedicated to the greatest breakthroughs and challenges of 21 ^st century developmental biology and reproductive medicine.