With the cooperation of Robert V. D. Campbell. This collection of technical essays and reminiscences is a companion volume to I. Bernard Cohen's biography, Howard Aiken: Portrait of a Computer Pioneer. After an overview by Cohen, Part I presents the first complete publication of Aiken's 1937 proposal for an automatic calculating machine, which was later realized as the Mark I, as well as recollections of Aiken's first two machines by the chief engineer in charge of construction of Mark II, Robert Campbell, and the principal programmer of Mark I, Richard Bloch. Henry Tropp describes Aiken's hostility to the exclusive use of binary numbers in computational systems and his alternative approach. Part II contains essays on Aiken's administrative and teaching styles by former students Frederick Brooks and Peter Calingaert and an essay by Gregory Welch on the difficulties Aiken faced in establishing a computer science program at Harvard. Part III contains recollections by people who worked or studied with Aiken, including Richard Bloch, Grace Hopper, Anthony Oettinger, and Maurice Wilkes. Henry Tropp provides excerpts from an interview conducted just before Aiken's death. Part IV gathers the most significant of Aiken's own writings. The appendixes give the specs of Aiken's machines and list his doctoral students and the topics of their dissertations.