Traditional approaches for treating severe crowded cases usually involve removal of some teeth with the intention to allow stable occlusion after the orthodontic treatment. However, in severe constricted cases, arch expansion could be the best approach to treat these cases without removal of permanent teeth. This chapter presents cases with severe teeth crowding and the possible treatment of these cases with clear aligners. This chapter presents moderate to severe crowding cases treated solely by clear aligners utilizing dental arch expansion. Also, this chapter presents diagnostic criteria for deciding expansion/non extraction or extraction approaches in detail.