I posted this info elsewhere (under a Canon P-150 thread) but thought it might help a lot of others that use TWAIN scanners in Mavericks. My experience is with Canon P-150M and Canoscan 8800F, but this works with others as well. Thanks to janegil at http://janegil.net for figuring this solution out, kraypsu here for alerting me to it, and Thorsten Lemke, developer of Graphic Converter for helping me as well. Basically, the TWAIN drivers Canon has are 32 bit. So although for me they work with the native Canon software, it's often less than desirable. They didn't work with Image Capture. I could start Graphic Converter in 32 bit, and scan, but some applications don't let you do that (I noticed that although Photoshop CS6 can do 32 bit, my version as installed is only 64). And some applications don't do TWAIN. So if you're having problems see here: http://janegil.net/2014/01/twain-scanners-in-os-x-maverick/ He figured out the missing ingredient, TWAINbridge. This can install it and your scanner will now appear in Image Capture as a device. You'd need a recent TWAIN driver in addition (Canon had ones circa 2012 for my scanners), and there are links there for TWAIN SANE drivers if you need them. And Photoshop requires a TWAIN plugin, BTW, for scanning BUT with TWAINbridge installed you can scan from the devices listed in Image Capture. Pretty sweet, no? And even cooler, I installed it in the Yosemite public beta. And got my 8800F to show up and scan in Image Capture as well.