"It's one of the big bets we're making," he said during the final stop of a farewell tour before he withdraws from the company's daily operations in July.
In five years, Microsoft expects more Internet searches to be done through speech than through typing on a keyboard, Gates told about 1,200 students and faculty members Thursday at Carnegie Mellon University.
Gates also said the software that is proliferating in various branches of science, including biology and astronomy, must become even more advanced.
"They're dealing with so much information that ... the need for machine learning to figure out what's going on with that data is absolutely essential," he said.
Microsoft is trying to establish ties not only with university computer science departments but also with researchers in other scientific areas "to help us understand where new inventions are necessary," Gates said.
Gates plans to retire as Microsoft's chief software architect in July and focus on philanthropy.