Spindle cell lesions arising in the breast represent reactive, benign, and malignant tumors with overlapping morphologic, clinico-radiologic, and immunohistochemical characteristics. Moreover, common entities comprising this subset are usually uncommon entities in overall prevalence. The combination of such diagnostic “disadvantages” can make the practicing pathologist feel uncertain from the onset of encountering such a case. We hope to dispel some of this discomfort by delineating a simple algorithm that provides structure and direction to the diagnostic work-up. Finally, we provide short summaries of the most commonly encountered mammary spindle cell lesions.