I had a possum in my backyard several times, and I'm in New York City.
At first I was concerned for my cats, since it's a fenced in backyard
where I let the cats roam when I'm home. However, I researched it and
possums are not a problem at all. They don't carry rabies or other
diseases, and they eat the stuff that you really don't want in your
yard such as damaging insects and slugs.

They're actually beneficial and get along with cats just fine. It has
been in my yard at the same time as my cats and they just ignore each
other. The cats are curious but, perhaps because they're about the
same size, they leave each other alone.

They're far better at climbing than my cats. The cats can't get out
(or don't try very hard) but the possum seems to have no problem. I've
never seen how he/she does it, because it is night when it's around
mostly, but it must actually cling upside down to the fencing that
tilts in around the yard and somehow gets around it.

Some time last year I noticed Nipsy and Espy in the hall looking at
something on the floor. One of them had brought in what turned out to
be a baby oPossum. I took it outside and stuck it through the chain
link fence into the groundcover in the next yard. A little while later
Nipsy was running around the yard with another one in his mouth. That
also went through the fence. It seems mom had let loose the litter and
there were quite a few babies around.

They are marsupials and have live births but then the babies mature in
the mother's pouch like a kangaroo.

By the way, if an opposable thumb is a sign of tool using, note that
the opossum has opposable thumbs on its front and back feet. One of
the babies that I put through the fence clung to it with its back foot
before dropping into the ground cover. Very weird to see.

All in all, leave possums alone or even feed them. They have a tough
life and are essentially harmless. Now, raccoons are another story.

Gee, I didn't know opossums were so unpopular. I guess they aren't as
cute as some animals, but once we found a mom and a couple of her
babies hanging by their tails in our lilac tree, and believe me, that
was darn cute.

Possums are harmless and actually beneficial because they keep rats
and other unwanted rodents away. When threatened they have no defense
but to show their big teeth and hiss, and if that doesn't work, they
play dead.

They come in our yard sometimes, usually at dusk. They don't bother
the cats and the cats don't bother them. Our neighbors used to worry
that the possums would bite their little dog, but when they called
Animal Control about having the possums removed, they were told to
leave them alone because their dog was a lot more likely to bite the
possums than they were to bother their dog.
-yngver