These are horizontal lines less than 2cm long, commonly found in the lower zone periphery.  These lines are the thickened, edematous interlobular septa.  Causes of Kerley B lines include; pulmonary edema, lymphangitis carcinomatosa and malignant lymphoma, viral and mycoplasmal pneumonia, interstital pulmonary fibrosis, pneumoconiosis, sarcoidosis.  They can be an evanescent sign on the CXR of a patient in and out of heart failure.