Mammography requires compression of the breasts to achieve the best quality images, because compression separates out normal tissue densities to see if an underlying cancer is present.  Good compression also prevents blurring of the image from heart pulsations and results in lower radiation exposure than with poor compression.  Most women would say that there is some discomfort with mammogram compression but that it is quick and isn't very painful.  The mammography technologist who does your mammogram should work with you to get good compression on your mammogram, but should back off if you indicate that you are actually having pain.  For premenopausal women with particularly tender breasts, try to schedule your mammogram at the time of month when the tenderness is least.  If you have concerns about the compression, discuss them with the technologist before the procedure.