Yep

Thanks---as you can see, we have a lot of it. We tried to reseed a bare patch in the spring and crabgrass grew in instead and spread. I have read that we should overseed in the fall and then put down a pre-emergent in the spring. When should we overseed? The crabgrass won't die off until the first frost, but by then wouldn't it be too late to overseed? I am just concerned that overseeding will be difficult because there is so much of it.

Since the crabgrass is so widespread, I would kill everything with round up, including what grass you have, then reseed in September.

There are many threads here that will guide you through the process.

Where do you live?

There is a reason you should seed in the fall and you nailed it. You now have a full crabgrass lawn.

Overseeding would be difficult. Joe has the right idea. Nuke it all and start over at the right time of year. Your location will determine when to start this. If you are in the UP of Michigan, then start now. If you are in Georgia, then wait a month. The idea behind early seeding in the fall is to get the grass up and growing in time to evaluate it and see if you need to overseed the new seed to make it more dense. Then by next summer the roots will have hardened off and the grass should be considered to be mature.

What kind of grass do you want? Is there shade in the area?