Mastitis represents a spectrum of inflammatory conditions. Lactational mastitis is the most common, with an approximate incidence of 10% in the United States, and it usually occurs in the first 3 months postpartum. Diagnosis is made clinically based on the presence of symptoms such as fever, malaise, focal breast tenderness, and overlying skin erythema or hyperpigmentation without the need for laboratory tests or imaging. However, obtaining milk cultures should be considered to guide antibiotic therapy, and ultrasonography should be performed to identify abscesses in immuno-compromised patients or those with worsening or recurrent symptoms. Because most cases of mastitis are caused by inflammation and not a true infection, a 1- to 2-day trial of conservative measures (i.e., nonsteroidal anti-inflammatory drugs, ice application, feeding the infant directly from the breast, and minimizing pumping) is often sufficient for treatment. If there is no improvement in symptoms, narrow-spectrum antibiotics may be prescribed to cover common skin flora (e.g., Staphylococcus, Streptococcus). Most patients can be treated as outpatients with oral antibiotics; however, if the condition worsens or there is a concern for sepsis, intravenous antibiotics and hospital admission may be required. Use of probiotics for treatment or prevention is not supported by good evidence. Factors that increase the risk of mastitis include overstimulation of milk production and tissue trauma from aggressive breast massage; therefore, frequent overfeeding, excessive pumping to empty the breast, heat application, and breast massage are no longer recommended because they may worsen the condition. The best prevention is a proper lactation technique, including a good infant latch, and encouraging physiologic breastfeeding rather than pumping, if possible.