US News & World Report provides a list of top US universities offering test-optional PhD engineering programs, including Stanford and MIT.
GradSchools.com lists PhD engineering programs in top US universities that do not require GRE scores, including University of California, Berkeley and Carnegie Mellon University.
MIT's official website outlines its test-optional admissions policy for PhD programs, including engineering, and provides guidance for applicants.
Inside Higher Ed publishes an article discussing the impact of test-optional policies on PhD admissions in top US universities, including those with engineering programs.
Peterson's provides a searchable database of PhD engineering programs in the US, including those that are test-optional or do not require GRE scores.
A video on YouTube discusses test-optional PhD engineering programs at top US universities, including University of Michigan and Georgia Tech.
A research paper from North Carolina State University examines the effectiveness of test-optional admissions policies for PhD programs, including those in engineering.
The National Science Foundation (NSF) provides information on test-optional PhD programs in engineering at US universities, including those funded by NSF grants.