Newborn screening is the process of testing newborn babies for treatable genetic, endocrinologic, metabolic and hematologic diseases. Robert Guthrie is given much of the credit for pioneering the earliest screening for phenylketonuria in the late 1960s using blood samples on filter paper obtained by pricking a newborn baby’s heel on the second day of life to get a few drops of blood.  Congenital hypothyroidism was the second disease widely added in the 1970s. The development of tandem mass spectrometry screening by Edwin Naylor and others in the early 1990s led to a large expansion of potentially detectable congenital metabolic diseases that affect blood levels of organic acids. Additional tests have been added to many screening programs over the last two decades. Newborn screening has been adopted by most countries around the world, though the lists of screened diseases vary widely.