The finite strip method is used to study the local, distortional, and flexural-torsional modes of buckling of I-beams bent about their major axis. The interaction between these modes is investigated. The buckling loads so determined are compared with those derived using the simpler but approximate theories normally used to design against specific buckling modes. The effect of lateral and torsional restraints on the buckling modes is investigated. A design chart for the local buckling stress of I-beams subjected to major axis moment is presented.