Breast cancer has various histological types that reflect not only morphological features but also biological characteristics. Therefore, it is not an exaggeration to say that breast cancers of different histological types are different diseases. It is generally accepted that the histological types of breast cancer are clinically significant because they serve as prognosticators and as the common language for improving the accuracy of clinical diagnosis. It is necessary to diagnose breast cancer at the level of not only histological findings by needle biopsy, but also the histologic type based on diagnostic imaging and cytological diagnosis. From the viewpoint of treatment, preoperative drug therapy is being performed more frequently to shrink tumors before breast-conserving therapy or to determine treatment sensitivity. The prognosis is favorable for patients who respond completely or patients in whom interstitial infiltration is completely eliminated histopathologically, and, as a result, it is important to assess therapeutic efficacy clinically and pathologically. Past experience has shed some light on differentiating cancers responsive to drug therapy from those unresponsive to drug therapy, as well as differentiating cancers in which therapeutic efficacy can be easily ascertained from those in which therapeutic efficacy cannot be easily ascertained. Preoperative drug therapy can be planned by making a histological diagnosis based on needle biopsy findings. Preoperative drug therapy is not indicated for noninvasive carcinoma and papillotubular carcinoma (invasive carcinoma with predominant intraductal components). While complete loss of interstitial infiltration can be expected with solid-tubular carcinoma, it cannot be expected with other histological types, such as invasive lobular carcinoma, adenoid cystic carcinoma, or metaplastic carcinoma (squamous-cell carcinoma and spindle-cell carcinoma). On therapeutic response assessment, the clinical and pathological findings generally match for solid-tubular carcinoma but not for scirrhous carcinoma and invasive lobular carcinoma. With mucinous carcinoma, mucus accumulation can remain, even though most cancer cells disappear; as a result, assessment based on tumor diameter changes is difficult. Histological diagnosis is also significant from the viewpoint of drug sensitivity, and it is important to maintain the accuracy of histological diagnosis.