This document presents a paper battery, which is a flexible, ultra-thin energy storage device made of paper and carbon nanotubes. Paper batteries combine the properties of batteries and supercapacitors. They are constructed by coating carbon nanotube ink onto paper, which acts as the separator between the lithium anode and carbon nanotube cathode. Paper batteries have advantages over traditional batteries as they are biodegradable, reusable, recyclable, durable, rechargeable, and leak-proof while being very lightweight and flexible. However, carbon nanotubes are expensive to produce and have low shear strength. Potential applications of paper batteries include smart cards, wristwatches, and wearable computers.