Combinatorial and learnability results are proven for projective disjunctive normal forms, a class of DNF expressions introduced by Valiant. Dedicated to Prof. Peter Hammer on the occasion of his 70th birthday. 1 Introduction The model of projection learning was introduced by Valiant [17], motivated by constraints imposed on learnability by biology. Projection learning aims to learn a target concept over some large domain, in this paper {0, 1} n , by learning some of its projections (or restrictions) to a class of smaller domains, and combining these projections. Valiant proved a general mistake bound for This paper represents an extended and revised version of the conference paper [16].