Tim Cook is set to leave his role as Apple CEO, bringing an end to a 15-year tenure in which he transformed the company’s operations while steering it through a period of enormous growth and change.

According to a report by Firstpost, Cook will move into the role of executive chairman, while John Ternus, Apple’s senior vice president of hardware engineering, is set to succeed him as CEO.

Cook joined Apple in March 1998 as senior vice president for worldwide operations, arriving at a company undergoing a major restructuring.

His expertise was not in industrial design or product development but in making large-scale operations more efficient.

He quickly began reshaping Apple’s manufacturing model.

Factories and warehouses were closed, while production increasingly shifted to contract manufacturers.

The strategy significantly reduced the amount of time products spent in Apple’s inventory, taking stock levels from months’ worth of products to only a matter of days.

Cook also adopted a long-term strategy for securing critical components.

His team invested in supplies such as flash memory before demand surged, helping Apple secure components that would later be used in products including the iPod Nano, iPhone and iPad.

The work gave Cook an increasingly important role in Apple’s wider business operations.

He later served as vice president for worldwide sales and operations before taking responsibility for the company’s operations in January 2007 and eventually becoming chief operating officer.

Cook’s importance became particularly evident during Steve Jobs’ periods of medical leave.

While Jobs remained involved in major decisions, Cook handled much of Apple’s day-to-day management, gaining valuable experience running the company at its highest level.

That experience helped make the eventual leadership transition less abrupt.

When Jobs resigned as CEO in 2011, Apple appointed Cook as his successor on August 24 of that year.

The change was significant, but Cook was hardly an outsider taking charge.

By then, he had spent more than a decade developing a deep understanding of Apple’s internal operations and had already played a major role in running the business.