Do you have tunnels in your lawn? Piles of dirt visible after the snow melts? That sounds like mole damage. While Moles don't do any real harm, they can make a lawn unsightly and create spots for weeds to germinate. If moles bother you, the best solution we've found is this product called Mole Chase.
The number of applications needed varies depending on how severe your mole problem is and how much they like your lawn. We'd recommend doing a few heavier applications towards the end of the season to prevent moles from causing damage over the winter, but treatments can be done anytime of the year. The bag says it will treat up to 6000 square feet but in our experience this is a product that works better with a heavier dose. We typically use one bag to cover 3,000 square feet. Visit FindLotSize.com to measure the square footage of your yard.
Mole Chase does work, in some lawns it takes more applications and heavier doses than others. It's the most effective way to handle moles (aside from trapping) that we've found.