Since the discovery of a scalar particle by the ATLAS and CMS experiments at the Large Hadron Collider, a major question in particle physics is whether this new state is the Higgs boson of the Standard Model, or one member of an extended Higgs sector predicted by several theories beyond the Standard Model. This seminar will review a selection of searches for additional Higgs bosons in the ATLAS experiment, based on the pp collision data collected in 2015 and the first half of 2016.