Alfred Harker FRS[1] (19 February 1859 – 28 July 1939) was an English geologist who specialised in petrology and interpretive petrography. He was lecturer in petrology at the University of Cambridge for many years, and carried out field mapping for the Geological Survey of Scotland and geological studies of western Scotland and the Isle of Skye. He and other British geologists pioneered the use of thin sections and the petrographic microscope in interpretive petrology.