  Warning: Many stupid statements and assumptions ahead. I have recently discovered that we have something of a bat problem.
They've probably been there for a year an a half, off and on. We noticed droppings around that time, but I didn't give it much thought, because the droppings were outside of the house. I thought that it might be squirrels or something, but wondered how they were getting to the attic vent, around which there was staining, indicating some activity. The guy painting the house at that time let me know what I was dealing with, when he told me that he saw a dozen or so fly out when he went up to paint that part of the house.
I figured that he scared them off, and that was the last we'd see of them. Yeah. Last weekend, when I noticed that the droppings had returned, and sprayed a hose up toward that attic vent, whereupon about 20 bats flew out, and bat guana ran down the side of the house like Willy Wonka's chocolate river.
I returned the next day with my hose, and chased the bats off again. I had hoped that being disturbed like that would cause them to leave for good.
Yeah.
The good news is that they're not in the attic; rather, they're hanging out on the outside of the screen, but protected by the slats outside that attic vent window on the side of the house. I went online and ordered some urlLink Bat-a-Way to put up in the attic. The smell supposedly drives the critters away.
We'll see. The other side is that I hate to drive them away completely, because they keep the mosquito population down (though, if you walk outside our house right now, you can't tell that they're doing a particularly good job). I may put a bat box on one of the trees, and see if I have any luck moving them there. 
