Progress was made more recently by Howe,  Moy, Gopal Prasad and Bushnell and Kutzko, who developed a theory of types and classified the admissible dual (i.e.
Recent breakthroughs in this field were made by Howe,  Moy, Gopal Prasad and Bushnell and Kutzko, who established a theory of classification and identified the permissible duals.