This is a phase II study of gemcitabine and Herceptin given weekly as a bolus infusion administered concurrently with radiotherapy in patients with regionally confined resectable or unresectable adenocarcinoma of the pancreas that overexpress HER2. Patients will be treated with external beam radiation in a standard manner over 6 weeks prior to surgical exploration. Gemcitabine and Herceptin will be administered on the first day of irradiation and weekly during the course of radiotherapy. Patients will be assessed for resectability after treatment and may undergo a pancreatic resection. Patients will then be treated with once weekly gemcitabine and herceptin for three weeks followed by 1 week of rest for up to 6 months. Patients will be assessed clinically for toxicity, tumor response, progression-free survival, and overall survival.