| 1987-present | Professor, University of South Florida, tenured 1988 |
| 1982-1989 | Professor with tenure, Arizona State University |
| 1980-1982 | Associate Professor, Arizona State University |
| 1978-1980 | Assistant Professor, Arizona State University |
| 1979-1981 | Associate Professor, McMaster University, tenured 1981 |
| 1976-1979 | Assistant Professor, McMaster University |
| 1964-1968 | Instructor, Cairo University |
| 2000-2001 | Visiting University Professor, City University of Hong Kong |
| 1999 | Visiting Member, Mathematical Sciences Research Institute, Berkeley (3 months) |
| 1996 | Visiting Professor and Leverhulme research fellow, Imperial College, London |
| 1990-1991 | Adjunct Professor, University of Toronto |
| 1990-1991 | Adjunct Professor, University of Toronto |
| 1990 | Visiting Professor, University of Paris VII (10 weeks in the summer) |
| 1988 | Visiting Professor, National University of Colombia (1 month) |
| 1987-1990 | Adjunct Professor, York University |
| 1987 | Visiting Professor, University of Alberta (1 month in the summer) |
| 1986 | Visiting Professor, University of Paris VII (10 weeks in the summer) |
| 1984-1985 | Visiting Professor, University of Minnesota, Minneapolis |
| 1982 | Visiting Professor, Kuwait University (winter and summer semesters) |
| 1976 | Visiting Scholar, Mathematics Research Centre, University of Wisconsin, Madison |
| 1975-1976 | Visiting Lecturer and Research Associate, University of Toronto |
| 1974-1975 | Assistant Scientist, Department of Mathematics and Mathematics Research Centre, University of Wisconsin, Madison |