Centrosomes are the organelles that organize microtubule networks and establish the bipolar mitotic spindle, which is essential for the segregation of chromosomes during cell division. Proper duplication of centrosomes is necessary to prevent genetic instability, thus control of this organelle is important in the suppression of tumorigenesis. The BRCA1 dependent ubiquitination activity regulates centrosome number in breast derived cell lines and this activity is likely critical for the tumor suppression activity of BRCA1. This review will focus on the importance of controlling centrosome number and on the effect of BRCA1 on the centrosome duplication cycle in mammary cells.