Invasive lobular carcinoma (ILC) is the second most common type of breast cancer, with a unique pathogenesis and distinct clinical biology. ILCs display a characteristic loss of E-cadherin, are largely estrogen receptor positive, HER2 negative, and low to intermediate grade. These features portend a favorable prognosis, but there is a tendency for late recurrences and atypical metastases. ILCs tend to be insidious and infiltrative, which can pose a challenge for diagnosis, and emerging data suggest they may have a propensity for a differing response to standard therapies.