The Head of Slytherin House is a Professor at Hogwarts School of Witchcraft and Wizardry who is in charge of looking after and disciplining the students of their House. The Head of House also serves as a Professor of a subject. Most, if not all, of the heads of Slytherin house attended Hogwarts and were sorted into Slytherin. It is unknown how a professor would be elected for the post, but it's possible that the headmaster chooses the professor who was sorted into Slytherin and embodies the qualities that house founder Salazar Slytherin possessed such as determination, resourcefulness, and cunning.