Abstract: This invention relates to sleeping bags, more particularly to sleeping bags whose outer shell, liner, and insulation characteristics may be varied through selection of various combinations that are particularly suitable for the planned usage. The sleeping bag of this invention comprises a case constructed from two panels whose sides and one pair of ends are attached to one another and the other pair of ends define an opening into the space between the panels. The sleeping bag further comprises an insulation layer that is sized and configured to be easily removable and replaceable through the opening in the case. At least one fastener attaches the insulating layer to the case. Insulation layers having different thermal ratings may be substituted as needed to obtain adequate protection under varying weather conditions. Being removable from the insulation layer, the case may be cleaned and/or aired separately therefrom.