Trespass to land is a substantial interference with a persons possessery interest in their land. Which causes substantial harm in the person being wronged. Trespass to land could be something as simple as someone walking across someones yard, it could be as simple as someone trying to dump toxic waste on someones property out in the boondocks. The bottom line is if someone goes on to your land, theyre trespassing and youre entitled to sue them. Sometimes it may just be a dollar that you could get as nominal damages because there was no real harm done. Sometimes it can be billions of dollars depending on whether it was radioactive waste dumped on your land.