Genistein is a well known protein tyrosine kinase inhibitor. It is structurally similar to 17- estradiol and exerts antiestrogenic effects. It also affects the signal transduction components Akt, FAK, ErbB-2 and Bcl-2. Key enzymes implicated in cancer invasion are also affected by genistein. A critical evaluation of the effects of genistein on breast cancer growth, signaling and gene expression is presented in this review.