-By Mike Shields, Mediaweek

NEW YORK Yahoo co-founder Jerry Yang will step down from his role as CEO, as the embattled Web portal said it has begun a search for his replacement.

Yang's impending departure was announced by Yahoo's board of directors late Monday. The 40-year old technology pioneer will aid in the search and will continue to serve on the board and in his previous advisory role as "chief Yahoo," said officials.

In June 2007, Yang took over the CEO title at the Yahoo board's request following the ouster of former CEO Terry Semel. His year-plus on the job has been nothing if not challenging, as Yahoo's stock price has languished and its once rapid revenue growth has slowed considerably. In addition, Yang's tenure has been marked by a drawn out, ultimately fruitless takeover attempt by Microsoft, which many have criticized the company for passing up.

And just recently, Yahoo saw a potential lifeline disappear, as Google pulled out of a proposed search ad partnership, bowing to pressure from the Department of Justice.

Yang's run as CEO has also been marked by numerous organizational changes, as well as a recently announced round of layoffs. However, Yahoo has had its share of successes as well, particularly as several of it's vertical properties continue to deliver huge audiences. In particular, Yahoo Sports drew the largest audience online during the recent summer Olympics, and Yahoo News scored major traffic surges during the just completed presidential election.

In a statement, Yang said that after helping move the company in a new direction, it was time for someone else to take it to the next level.

"When the board asked me to become CEO and lead the transformation of the company, I did so because it was important to re-envision the business for a different era to drive more effective growth," he said. "Having set Yahoo on a new, more open path, the time is right for me to transition the CEO role and our global talent to a new leader. I will continue to focus on global strategy and to do everything I can to help Yahoo realize its full potential and enhance its leading culture of technology and product excellence and innovation."

Yahoo has hired the executive search firm Heidrick & Struggles to assist in the CEO search process, which will include both internal and external candidates, said officials.