Devices, systems and methods for dynamically stabilizing the spine are provided. The devices include an expandable spacer or member having an unexpanded configuration and an expanded configuration, wherein the expandable member in an expanded configuration has a size, volume and/or shape configured for positioning between the spinous processes of adjacent vertebrae in order to distract the vertebrae relative to each other. The systems include one or more expandable members and a mechanical actuation means for expanding the expandable member. The methods involve the implantation of one or more devices or expandable spacers.