We propose and solve a simple fault growth model describing the powerlaw dependence of the fault length distribution observed both in nature and in laboratory models of the lithosphere. Starting from an arbitrary initial fault length distribution and assuming that the rate of growth of a fault is proportional to a power of its size, we show that the fault length distribution evolves towards a universal powerlaw curve. Comparison between this theory and experimental results is encouraging. ¿American Geophysical Union 1991