You may think this is a no-brainer topic but believe me it's not. Holding a chef's knife incorrectly can cause wrist pain and will tire you more quickly. First let's look at the proper hold for chopping.

The thumb and index finger are on the blade and the last 3 fingers grasp the handle. This hold gives you more control over the knife. When chopping you must bear down on the knife to make your cuts. Holding the knife like this puts your wrist power closer to the item being chopped, hence it is easier to use.

Lastly, all good chef's knives are balanced. This knife's center of gravity is at the bolster. The bolster if the part of the knife between the handle and where the blade begins to take it's flat shape (Shown to the right between the two blue lines). 

Now notice above that the bolster is right at the 2nd finger; this is the power center of your hand. You wind up with the knife weight evenly distributed (comfortable) and your wrist/hand power is at its' greatest.

This is a forged knife which has a bolster. A Stamped knife has no bolster, but, you would hold it the same way.

Now keep in mind that this is the method for chopping. When using the knife for other tasks you may need to slide your hand back to grasp only the handle.

Here's a video clip I found that shows the technique. Notice the hold, and also notice the index finger of the hand holding the Zucchini. It curves slightly in towards the hand. This method keeps your fingertip protected from the knife's edge.

Click Here for the video.