Changes in the microenvironment are important in the development of cancer and further tumor growth. Although landmark discoveries have been made regarding genetic alterations in cancer at a cellular level very little is known about protein regulation in the extracellular space. In the microenvironment many growth factors are activated at a post-translational level by interactions of different cell types such as epithelial cells, fibroblasts, adipose cells, and immune cells. The extracellular space is the bioactive site for the majority of growth factors and increased knowledge of protein activation in this compartment is of utmost importance for our comprehension of tumor biology. Microdialysis is a minimally invasive technique, which enables sampling of molecules in the extracellular space. It is applicable in human cancer as well as in experimental tumors. This review describes microdialysis, its application and the up to date literature of microdialysis for detection of growth factors in cancer with special emphasis on breast cancer.