A while ago I gave a talk on "Categorified Gauge Theory" at the 2002 Joint Spring Meeting of the Pacific Northwest Geometry Seminar and Cascade Topology Seminar. I've just put the transparencies for this talk on my website: http://math.ucr.edu/home/baez/gauge/ Here's the abstract: In electromagnetism we can think of the vector potential as a 1-form A which couples to charged point particles in a very natural way - we simply integrate it over the particle's worldline to obtain a term in the action. Similarly, in string theory there naturally arises a 2-form B, the Kalb-Ramond field, which we integrate over the string worldsheet. The resulting theory of "2-form electromagnetism" is formally very similar to Maxwell's equations: in particular, we define a curvature 3-form G = dB and require that *d*G = J where the current J is now a 2-form. Just as the electromagnetic vector potential should really be regarded as a connection on a U(1) bundle, the Kalb-Ramond field should really be thought of as a connection on a "U(1) gerbe". Moreover, just as U(1) bundles are classified by the 1st Cech cohomology with coefficients in the sheaf of smooth U(1)-valued functions, U(1) gerbes are classified by the 2nd Cech cohomology with coefficients in this sheaf. Electromagnetism can be generalized to Yang-Mills theory by replacing the group U(1) by an arbitrary compact Lie group. This raises the question of whether we can similarly generalize 2-form electromagnetism to some sort of "higher-dimensional Yang-Mills theory". We show how to do this by categorifying the concepts of smooth manifold, Lie group and Lie algebra, and setting up a theory of bundles, connections and curvature in this new context. In particular, we define a "Lie 2-group" to be a category C where the set of objects and the set of morphisms are Lie groups, and source, target, identity and composition maps are homomorphisms of Lie groups. This turns out to be the same as a "Lie crossed module": a pair of Lie groups G and H with a homomorphism t: H -> G and an action of G on H satisfying the equations in the usual definition of crossed module. Just as a connection on a trivial G-bundle is the same as a Lie(G)-valued 1-form, a connection on a trivial C-2-bundle turns out to be a Lie(G)-valued 1-form together with a Lie(H)-valued 2-form. Following ideas of Breen and Messing, we give formulas defining the curvature of such a connection, which consists of a Lie(G)-valued 2-form together with a Lie(H)-valued 3-form. We write down the obvious generalization of the Yang-Mills action for a connection on a trivial C-2-bundle, and derive the "categorified Yang-Mills equations" from this action. We also show that in certain cases these equations admit self-dual solutions in five dimensions. We conclude by sketching how nontrivial C-2-bundles can be classified by the 2nd nonabelian Cech cohomology.