On the city buses in Aarhus and Aarhus Letbane, you must have purchased your ticket in advance. Here you cannot buy a ticket once you have boarded.

But you can buy your ticket from the driver in Midttrafik's blue buses. However, it will always be an advantage to have the ticket in place in advance, either via apps or by purchasing from Midttrafik's webshop.

See more here (danish).