Some of them will fence in 100 acres. Ought to be enough. Most give a tone as the dog approaches a boundary. If it keeps going it will get "stimulation". The range is adjustable to some degree. Almost all the collars have varying levels of stimulation. Sportdog for example has seven levels ranging from just vibrating the collar up to 6 more levels of stimulation. The dogs are pretty easily trained to the boundary using white flags as the visual. I know a lot of people that use them and like them.