History of Programming Languages (HOPL) is an infrequent ACM SIGPLAN conference. Past conferences were held in 1978, 1993, and 2007. The fourth conference was originally intended to take place in June 2020, but was postponed to 2021. HOPL I HOPL I was held June 1 – 3, 1978 in Los Angeles, California. Jean E. Sammet was both the general and program committee chair. John A. N. Lee was the administrative chair. Richard L. Wexelblat was the proceedings chair. From Sammet's introduction: The HOPL Conference "is intended to consider the technical factors which influenced the development of certain selected programming languages." The languages and presentations in the first HOPL were by invitation of the program committee. The invited languages must have been created and in use by 1967. They also must have remained in use in 1977. Finally, they must have had considerable influence on the field of computing. The papers and presentations went through extensive review by the program committee (and revisions by the authors), far beyond the norm for conferences and commensurate with some of the best journals in the field. Preprints of the proceedings were published in SIGPLAN Notices (volume 13, issue 8, August 1978). The final proceedings, including transcripts of question and answer sessions, was published as a book in the ACM Monograph Series: History of Programming Languages, edited by Wexelblat (Academic press, 1981). HOPL II HOPL II was held April 20–23, 1993 in Cambridge, Massachusetts. John A.N. Lee was the conference chair and Sammet again was the program chair. In contrast to HOPL I, HOPL II included both invited papers and papers submitted in response to an open call. The scope also expanded. Where HOPL I had only papers on the early history of languages, HOPL II solicited contributions on: early history of specific languages, evolution of a language, history of language features and concepts, and classes of languages for application-oriented languages and paradigm-oriented languages. The submitted and invited languages must have been documented by 1982. They also must have been in use or taught by 1985. As in HOPL I, there was a rigorous multi-stage review and revision process. Preprints of the proceedings were published in SIGPLAN Notices (volume 28, issue 3, March 1993). The final proceedings, including copies of the presentations and transcripts of question and answer sessions, was published as the ACM Press book: History of Programming Languages, edited by Thomas J. Bergin