Recent remarkable progress in hormonal therapy has provided great benefit to breast cancer patients, but it also evokes novel issues: how accurately can the efficacy of each hormonal therapy be predicted and how can hormonal therapy-resistant patients be treated? These clinically important issues must be closely related to the biological events in each cancer, such as the alteration of intracellular multiple estrogen signaling pathways and the estrogen-related cancer microenvironment, which has recently revealed by molecular biological studies on estrogen and its receptors. However, the estrogen signaling status in individual breast cancers has not been clarified yet. Here we present the context of these issues and introduce our study of new tools which enable the visualization of estrogen signals in individual cancers. The assessment of estrogen receptor (ER)-alpha activity in individual cancers or ER-activating ability of the cancer microenvironment in each breast cancer patient revealed several new findings and interesting observations. We hope that these approaches provide new clues about the estrogen-dependent mechanisms of breast cancer development, and will be useful to advance the diagnosis and treatment of breast cancer patients.