Take the following example. Let's say we want all the blue lines to terminate at the red line.
![Image](http://img137.imageshack.us/img137/4893/hdtut001ka3.png)
First we need to select all the blue lines. Notice the dashed line around my selection window - this means only objects fully within this box are selected. This is achieved by holding down the Option key while using the Selection tool.
![Image](http://img137.imageshack.us/img137/59/hdtut002nt5.png)
Next we choose the Extend Tool in the Edit bar. If, like me you prefer keyboard shortcuts to pushing buttons, then activate Extend by pressing +E.
![Image](http://img137.imageshack.us/img137/8455/hdtut003al4.png)
Next, click on the red line...
![Image](http://img137.imageshack.us/img137/1044/hdtut004xm8.png)
And voilà!
![Image](http://img301.imageshack.us/img301/4782/hdtut005uw1.png)
Cheers.