The effects of adding wheat and oat bran, at different levels and milled to varying particle size ranges, to dough formulations were investigated, with respect to aeration and dough development during mixing, expansion during proving and final baked loaf characteristics. Addition of bran reduced the density of the dough at the end of mixing, largely because of the increased water absorption, with bran particle size having little effect on dough density. Wheat bran substituted for flour at 7.5% increased the capacity of the doughs to expand during proving, but despite this the baked loaf volumes were lower, and even lower at 15%. Grinding the wheat bran had little effect on optimum work input, water absorption and expansion during proving, but gave slightly lower loaf volumes and a finer crumb texture. The low correlation between proving expansion and baked loaf volume suggested that the effects of the wheat bran in bread formulations occurred during baking. By contrast, adding oat bran reduced both the maximum expansion capacity of the dough during proving and the baked loaf volume, suggesting that the oat bran exerted its effects largely during proving. Grinding the oat bran gave lower optimum work inputs and higher water absorptions, and reduced both proving expansion and baked loaf volumes.