We caught the very end of the local Monarch Watch butterfly tagging this morning. The butterflies congregate heavily in a local wetland area, and for one day in September each year the Monarch Watch folks provide tags and nets for people to come out and tag.
The tags are just little stickers--they don't hurt the butterflies or keep them from flying. Each tag has a unique number, and when the butterflies make it to Mexico--where they migrate to in the winter--people there catch them and report the tag numbers back to Monarch Watch. In addition to using the information for their research, they then put the numbers online, so pretty soon I'll be able to check and see if the butterfly I tagged actually made it safely to Mexico.
If you're interested in raising caterpillars or tagging butterflies or just about anything else related to Monarchs, I highly recommend checking out the Monarch Watch site. In addition to having loads of information, they sell tagging kits and butterfly gardening supplies, and the proceeds go to help with their research.