If a certain area of concern is found by the mammogram, ultrasound is a good test to look at that. It also helps to tell the difference between cysts and solid masses without using a needle to draw out fluid.
When the tests show that you might have breast cancer, biopsy is done, because it's the only way to know for sure.
There is an incidence of about 15% false negative mammogram readings (the mammogram is negative, although the doctor has felt a discrete breast lump)
If this is a case, biopsy of a discrete lump needs to be performed.