So, I'm starting to read World War Hulk. I was wondering, why doesn't Hulk transform to Banner? Is it because Banner and Hulk were separated? How does that work? Did Banner stay on Earth? When do Banner and Hulk become one again? In Marvel NOW they seem to be the same being.

He doesn't transform into Banner because (if you're in Planet Hulk), he's on a hostile alien planet with no spare tech around that Banner is familiar with, is constantly under attack by people and things capable KILLING him, and even in quiet moments, is surrounded by people who trust him, but may not if they know Banner. The Hulk doesn't trust Banner, just like Banner doesn't trust Hulk.

If you're in World War Hulk, he is WAGING A WAR. Even though him and Banner are allied, there isn't a great deal Banner can do aside from get taken out if Hulk transforms back.

@jonsmith: Wait, so Hulk can control his transformations? Can Banner control the transformations?

Probably a combination of factors: Banner can will the transformation into Hulk when he needs to, and Hulk can take a breather to transform back. Obviously, due to the aforementioned distrust between the two (and Hulk's off-and-on savagery), neither usually does this unless a situation specifically calls for the others abilities. Due to the aforementioned circumstances, Banner isn't really of great use on this planet, so Hulk has no reason to transform back. In addition, he's been exiled and betrayed by those he trusted, not to mention that the area he's been exiled to is constantly trying to kill him, so he has absolutely no moment where he calms down enough to warrant a transformation back.

Usually when the Hulk sleeps, he transforms into Banner, but since Banner isn't in his usual hurry to step up, that doesn't happen here.

Look at it like this: Normally, Banner and Hulk are constantly at war, because Hulk causes a lot of damage, and Banner is constantly trying to destroy or restrain Hulk. Hence the transformations between the two. Here, the two have a kind of truce since Banner's just as betrayed as Hulk. So Hulk can run rampant without worrying about transformations back and forth.

@jonsmith: Oh, I get it, thanks!

I forget how exactly he did it, but during the planet hulk storyline the hulk gained control of himself. But after the events of planet hulk the Hulk hit a entire new level of rage, however he did revert once or twice during WWH if im not mistaken. I know for a fact that he did on his way to earth cause one of the warbound walked in on him.

He didnt revert back to banner by choice because banner didnt just lose his wife and child, the Hulk did.

True that. When he's angry or stressed by any other means He turns into the Hulk.

Then if the Hulk gets angry or stressed by any means, he gets stronger.

Being pushed towards his limits makes the Bruce Banner/Hulk stronger.

His strength rises to whatever the measure of force that is pushing him to his limits.

The extra-chemical that allows him to draw mass and energy into himself to any degree, makes him indestructible monster of limitless power.