A retaining wall is a specific application requiring footings, etc. if the wall is simply 1-1.5ft, it is unlikely to be a retaining wall. If it is, you'll probably want professional help. If it's just a garden wall, there are a range of options from loose stacked through to very formal cut and mortared. It depends on the stones, the look you want and how much effort and continued effort you want to expend. Options might include a professional stone mason, but if you're keen on the DIY, grab a book from the library and go at it.

Thanks, ST—I am consulting professionals, who are giving me different opinions, so I thought I'd educate myself a bit. Your suggestions are helpful. The walls, btw, are retainers for two raised garden beds, each with soil sloping up to form berms on either side of the walkway. We have a narrow Bklyn back yard and wanted to insert some variation in topography. After 15 years, the walls are a bit wobbly in places and, so, need stabilizing—the stones are irregularly shaped. One person said he'd have to dismantle both walls and start from scratch. Two others are confident they can tuck in stones and other stabilizing material where needed. Thanks again for taking time to reply.

Retaining walls require footings, depending on the soil conditions these may be anywhere from 18 inches to several feet deep. Generally retaining walls built of stone use mortar to keep them from shifting. Stacked stone walls are completely inappropriate for holding soil back the way a retaining wall does. They will shift and do not have the interior support required to hold the weight back. The person who said they'd have to dismantle them and start from scratch is giving you the answer that is best long term. http://www.thisoldhouse.com/toh/article/0,,219681,00.html

Thanks for posting the link Jessica. From reading it, I notice that stacked walls of mortarless stone should be no higher than 3 feet. Our walls vary from 12" to 18" at the most, and after 10 years, only a few of the top stones are wobbly, so I think I'll try the tuck-in method first.