A mammogram is an x-ray picture of the breasts. It is used to find tumors and to help tell the difference between noncancerous (benign) and cancerous (malignant) disease. One breast at a time is rested on a flat surface that contains the x-ray plate. A device called a compressor is pressed firmly against the breast to help flatten out the breast tissue. Each breast is compressed horizontally, then obliquely and an x-ray is taken of each position.