Breast cancer is a heterogeneous disease that afflicts all patients differently, and therefore requires individualized treatment depending on a large variety of factors. Several methods of classification exist to divide patients into meaningful groups in order to better personalize their treatment regimens. Healthcare is evolving into more use of personalized treatments that can more effectively treat patients on an individual level, rather than by using more generalized treatments that may not prove effective in all patients. In addition, personalized treatment also aims to reduce adverse effects, while increasing effectiveness. Estrogen receptor (ER) status is one such method of grouping breast cancer patients into different treatment groups. Based on stage diagnosis and determination of receptor status, initial treatments such as surgery or radiotherapy may be used. Standard chemotherapy is another method, however, side effects may vary among patients and may be quite adverse. Other treatments include hormone or receptor blocking. This thesis has identified an alternatively spliced variant of classical ERα that resides in the plasma membrane of breast cancer cells and plays a major role in rapid signaling by estrogen. The overall aim of this thesis was to examine the role of the membrane receptor for 17β-estradiol (E2) in breast cancer that enhances breast tumor aggressiveness and to evaluate the mechanisms by which it functions. The general hypothesis was that nonclassical estrogen signaling through the proposed membrane-associated ER, ERα36, can promote breast tumor aggressiveness by enhancing cell survivability while altering expression of angiogenic and metastatic factors. This work examined the mechanisms of ERα36-dependent signaling in breast cancer cells, and the correlation of ERα36 to clinical outcome in human breast cancer tissue through histological evaluation. These data provide significant research as they provide a greater understanding of estrogen signaling in breast cancer through ERα36 and its role in tumorigenicity and metastasis. This study also proposes further clinical examination of ERα36, and suggests drug design to target ERα36 followed by preclinical studies to determine if drugs targeting ERα36 would benefit breast cancer patients by reducing tumorigenicity and increasing survival.