The present invention provides a modular stent-graft system. In one embodiment, a prosthesis comprises a first tubular graft comprising a layer of graft material, one lumen extending therein, and a first fenestration extending through the layer of graft material. A layer of fenestration covering material attaches to the layer of graft material. The layer of fenestration covering material is disposed in the lumen of the first tubular graft and partitions the first fenestration from the lumen of the first tubular graft. A first non-stented opening is disposed proximal to the first fenestration and communicates with the first fenestration between the layer of graft material and the fenestration covering material. In use, a proximal end of a second tubular graft sealably engages the first non-stented opening, and the second tubular graft further extends through the first fenestration and into a branch vessel.