There are many reasons for trees to drop leaves mid-summer. Sometimes it indicates need for water, or disease problems, or pests, though not always. You need to know what kind of trees you have. Are you able to send photos (near and far) and describe the growing situation (drainage, light exposure, soil type)? Do you see similar trees on properties around you doing the same thing?

I've attached photos of the trees, including a close-up. The two trees are the same type, I think. I don't know the soil type. They are pretty much in full sun most of the day and on flat ground. I don't see similar trees on surrounding properties. The trees are really dense -- lots of branches criss-crossing each other -- so I don't think they've been pruned in a while (just a guess, though, since I know nothing about trees). These trees are the only shade in my yard, so I don't want to lose them. Thanks for any help.

These appear to be Magnolia trees, though I'm not 100% sure. Did they have leaves in the winter? Flowers? They don't seem to need pruning.
They are drought-stressed though. Trees and shrubs don't do well without summer watering at least occasionally, as we are dry here for so many weeks that in most areas, there is no deep water for tree roots to access. Light daily sprinkling won't help much either. I recommend you run a sprinkler low, or just a hose at a low amount, so water soaks into the root zone deeply every week or so. Use a shovel the next day to judge the depth if possible. Your goal for trees is about 2 feet deep, as mentioned in this article about summer watering from OSU Extension, http://extension.oregonstate.edu/gardening/easy-ways-conserve-water-garden-during-dry-year.  It might be an hour or more to get the soaking done.

Thanks so much for the info, Jacki. I think you're right -- that they are magnolias. I bought the house and moved in at the end of April, so I don't know if the trees had leaves in the winter, but I do remember that they had flowers when I moved in. I will give them a deep watering every couple of weeks and hopefully that will take care of the problem. Thanks!

Your tree may continue to drop leaves this summer, but overall the appearance should improve. Next year keep it healthy and you'll see if pruning is warranted, or if other health issues exist.