This phase II, open-label, randomized study will evaluate the efficacy and safety of MEHD7945A versus cetuximab in patients with recurrent/metastatic squamous cell carcinoma of the head and neck who have progressed during or following platinum-based chemotherapy. Patients will be randomized to receive either MEHD7945A 1100 mg intravenously (iv) every 2 weeks or cetuximab 400 mg/m2 iv loading dose followed by 250 mg/m2 iv weekly. Patients treated with cetuximab (Arm B) may cross-over to MEHD7945A (Arm A) upon central confirmation of progressive disease and upon meeting eligibility criteria. Anticipated time on study treatment is until disease progression or intolerable toxicity occurs.