A linear code C is called self-dual if C = C⊥. Clearly the rate of such a code is 1/2. Many authors have studied such codes and discovered interesting connections with invariant theory and with lattice sphere packings (cf. Mac Williams and Sloane, 1977, Ch. 19). Recently there has been interest in geometric Goppa codes that are self-dual. For examples see alg.geom. § III. ref. [5], [6]. Here we give some theorems about self-dual codes.