One of the most important topics in evolutionary biology concerns the origin and evolution of sex-determining systems and sex chromosomes. Certain plant species display younger sex-chromosome systems in different evolutionary stages. It is thought that the same evolutionary forces described for animals are operating in plants. However, in opposition to the situation in animals, sex-determining mechanisms seem to be more flexible, and most species with separate sexes have evolved directly from ancestors with both sex functions. These features make plants excellent models for studies on sex determination. In this context, early evolved plant-sex chromosomes have given rise to many studies in recent years. In this book, the most recent findings are highlighted and reviewed, focusing specifically on model species, including Carica papaya, Fragaria virginiana, Silene latifolia and Rumex acetosa