Let V be a vector space of dimension d over Fq, a finite field of q elements, and let G≤GL (V)∼=GLd(q) be a linear group. A base
for G is a set of vectors whose pointwise stabiliser in G is trivial. We prove that if G is a quasisimple group (i.e. Gis perfect and G/Z (G) is simple) acting irreducibly on V, then excluding two natural families, G has a base of size at most 6. The two families consist of alternating groups Alt m acting on the natural module of dimension d=m−1 orm−2, and classical groups with natural module of dimension d over subfields of Fq.