Abstract: A propellant pillow for use in an implantable pump is described herein. The propellant pillow generally includes a propellant bag having a septum attached thereto. In one embodiment, the pillow may further include a rigid member associated with the propellant bag to prevent piercing of the bag by an injection device inserted through the septum during a filling process. In another embodiment, the propellant pillow includes a propellant bag having a surrounding layer of resealable material encompassing the bag. The surrounding layer also serves to prevent piercing of the bag during filling of the pillow with a propellant. Methods of filling such propellant pillows with propellant as well as utilizing such pillows in connection with implantable pumps are also described herein.