Arthur Shackelton Campbell was an American protistologist active mostly in the first half of the 20th century. To those of us who work on ciliates, he is known as the second author of two classic monographs on tintinnid ciliates with Charles A. Kofoid. To those of us who work on microfossils, or on rhizaria, he is known for some of the first American monographs on fossil radiolarians with Bruce L. Clark, and articles on fossil foraminifera with Joseph A. Cushman, as well as for chapters on fossil radiolaria and tintinnids in Raymond C. Moore's treatise on invertebrate paleontology. Arthur Campbell worked then with major figures of his time, but is little known in his own right. This is perhaps because he worked successively on very different taxa. He began writing as an undergraduate on a variety of invertebrate taxa, then focused on tintinnid ciliates, followed by fossil foraminifera, and finally on fossil radiolarians. Here a summary of his professional life and varied scientific contributions are given.