Bananas are safe for dogs. The main caveat is that they're on the higher end of the sugar spectrum for dog-friendly fruits — so a few slices as an occasional treat, not half a banana every day.
On the upside, bananas have some solid nutrients: potassium, magnesium, vitamin B6, vitamin C. They're one of the softer fruits, which makes them easy for older dogs or dogs with dental sensitivities. And most dogs will eat them — that slightly sweet, dense texture seems to go over well.
The peel question
Banana peel isn't toxic to dogs, but don't let them eat it. The peel is fibrous and tough, and a dog's digestive system isn't built to handle it — it can cause stomach upset or, in smaller dogs, create a blockage. If a dog swipes a peel off the counter and takes a bite, it's probably fine. If they ate the whole peel, watch for signs of GI distress and call your vet if anything seems off.
Portion sense
A couple of banana slices for a medium or large dog, one small slice for a small dog. Frozen banana slices are a popular option — most dogs like the cold and the firmer texture, and it's a convenient way to store them (slice a banana, freeze the slices, pull out a couple as needed).
If you want a lower-sugar fruit option that's just as easy to use as a treat, blueberries are hard to beat — small, no prep needed, and nutritionally solid. Watermelon is another one that's lower in sugar and a huge hit with most dogs.
Bananas are fine. Just don't make them a staple — a bit of variety in treat choices is better anyway.