A fundamental problem in wireless networks is to develop communication protocols that achieve high throughput in the face of noise, interference, and fading, all of which vary with time. An ideal solution is a rateless wireless system, in which the sender encodes data without any explicit estimation or adaptation, implicitly adapting to the level of noise or interference. In this paper, we present a novel rateless code, the spinal code, which uses a hash function over the message bits to produce pseudo-random bits that in turn can be mapped directly to a dense constellation for transmission. Results from theoretical analysis and simulations show that spinal codes essentially achieve Shannon capacity, and out-perform best-known fixed rate block codes.