The Hayward fault is a right-lateral, strikeslip fracture in the crust to the east of San Francisco Bay. Its earthquake history shows activity for the last 130 years; in 1836 and 1868, large damaging earthquakes were accompanied by surface rupture along the fault. At Berkeley, an intensive special study of the mechanics of the fault is possible. A mining adit exposes the gouge clay of the shear zone. Differential displacement of a culvert across the fault, measured continuously since 1965, shows a slippage rate of 2.5 mm per year over all with a definite pattern of seasonal fluctuations. The monitoring of small local earthquakes shows an average of 3 to 5 per year (2<M<3) within 10 km of Berkeley; focal depths are quite shallow with a few as great as 10 km.