Abstract: An adjustable helmet includes a main shell having a first side portion, and a second side portion. The first side portion and the second side portion are laterally flexible with respect to one another. The adjustable helmet includes a secondary shell coupled to the main shell at a location between the first side portion and the second side portion. At least a portion of the first side portion and at least a portion of the second side portion each extends over at least a portion of a top surface of the secondary shell. The adjustable helmet includes a tensioning device. A first end of the tensioning device is coupled to the first side portion and a second end of the tensioning device is coupled to the second side portion so as to provide a tension force between the first side portion and the second side portion.