  We derive harmonic superspaces for N=2,3,4 SYM theory in four dimensions from
superstring theory. The pure spinors in ten dimensions are dimensionally
reduced and yield the harmonic coordinates. Two anticommuting BRST charges
implement Grassmann analyticity and harmonic analyticity. The string field
theory action produces the action and field equations for N=3 SYM theory in
harmonic superspace.
