How Phil Knight built Nike into of the biggest brands in the world and became a billionaire

Over the past 51 years, Nike co-founder Phil Knight built the company from an idea he had in grad school into one of the biggest companies in the world.

And with a net worth of $21.6 billion, he's also one of the world's richest self-made billionaires.

Knight might be stepping down as the brand's longtime chairman this year, but his legacy will live on.

Here's a look back at how Knight's marketing prowess and relentless spirit have driven Nike from a selling a single running shoe to being synonymous with athletic gear.