How do I copy my music library from my iMac to my Mac Book Pro?

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If you only want to transfer music and both computers are on the same network, the easiest way is to use Home Sharing > http://support.apple.com/kb/ht3819

If you want to transfer more iTunes data, you can consider the option of transferring the whole iTunes library by making a backup onto an external drive > http://support.apple.com/kb/ht1751 Do the "Restoring your iTunes Library backup" step on your MacBook Pro. If you follow these steps, you will have the same iTunes library on both computers.

If both Macs have got a FireWire cable, you can connect them with a FireWire cable and put the iMac in Target Disk mode, so you will be able to access to the iMac's hard drive from the MacBook Pro.

As you can see, there are a lot of options, but I recommend you the iTunes library transfer through an external drive. After transferring the iTunes library, you can use the external drive to make backups of your computers using Time Machine > http://support.apple.com/kb/HT1427