A thick lawn is your best defense.
Weeds are opportunists. They will find bare spots or places where your grass is weak, and they will exploit them to their advantage.
Perennial weeds (weeds that grow from their roots every year) can spread and make a lawn unsightly. Annual weeds (weeds that die at the end of the season and reseed the next year) can leave bare spots that are vulnerable to runoff.
No matter what weeds you have, the first line of defense is preventive practices. Try these options to get at the root of the problem first, before resorting to herbicides.
Mow high. Do not mow grass shorter of lemon berry candy og than recommended for the species you grow. Mowing at 3 inches or higher helps grass shade out weeds and encourages a thicker, more competitive turf. See other sections of this site to make sure that you are using the right grass species, fertilizing and watering correctly, and generally doing all you can to encourage healthy grass.
Reduce compaction. Pay special attention to heavily used areas and sections next to pavement. Weeds can gain a foothold in these spots and spread to the rest of the lawn if it is weak.