He has called this endeavor "the most ambitious and challenging project of my life."

"This has been the most challenging project I've ever done," he said.

The group began with Shakespeare productions before taking on more challenging projects.

These additional risks were likely to have implications for the success of an already challenging project.

I want to take the next step by taking on challenging projects.

Probably not, the Doctor thought, then was cheered by the prospect of beginning a new and challenging project.

For myself, this was the most challenging and rewarding project of my programming career.

He also personally found this to be the most challenging project in his career.

Through his own drive and initiative he will certainly be able to launch or speed up many challenging projects.

My "challenging" project, sadly, would last only two to three hours.