This paper proves conjectures originating in the physics literature regarding the intersection exponents of Brownian motion in a halfplane. For instance, suppose that B and B ′ are two independent planar Brownian motions started from distinct points in a half-plane H. Then as t → ∞, P B[0, t] ∩ B ′ [0, t] = ∅ and B[0, t] ∪ B ′ [0, t] ⊂ H = t −5/3+o(1) .