The placenta is a transient organ of reproduction unique to eutherian mammals. Being gestational in origin, it shares only half of its genome with the maternal uterus. Although it exists for just 9 months, the placenta undergoes constant morphological and biological changes, both within its structure and at its interface with the maternal endomyometrium at the implantation site. It is at this interface that complex processes, such as fetomaternal tissue remodeling, nutrient and gas exchange, hormonal production, and immunological interactions, are meticulously regulated. These processes maintain a delicate balance, ensuring that maternal support is provided to the rapidly developing embryo while preventing maternal immunological rejection of the growing placenta. Proliferative disorders, including tumors arising from the placenta, possess distinct genetic, biological, and immunological characteristics that differ significantly from those seen in somatic and germ cell disorders. Genomic imprinting—the preferential inactivation of one of the two parental gene expressions in the placenta—represents a key feature of mammalian biology and plays a critical role in the pathogenesis of gestational trophoblastic diseases.