On a smooth surface, rolling wheels are the best way to get around. But it’s rocky on Mars. The Mars robotic rover would sometimes get hung up for days when a rock as big as itself blocked its way.

Getting a robot to walk as well as a person has long been a robot scientist’s dream. While you walk, you constantly shift your weight as you swing your arms. You make adjustments to your balance, posture, the length of your stride, and the force of your step. It’s not easy to design a robot with the brainpower, sensors, and flexible joints to do all that. But friendly, four-foot-tall ASIMO comes close.

To help it stay balanced, ASIMO has a speed sensor and soft projections on its feet that act like toes. ASIMO can stand on one foot, turn corners without shuffling, walk smoothly up and down stairs, and even dance a little.

Why walk or roll? Lots of animals hop, and this frogbot does, too. It doesn’t always land on its “feet” (neither do grasshoppers), but it knows how to right itself and get hopping again. Hopping robots might replace wheeled rovers in space exploration. The bouncy bots could easily hop over the rocky surface of planets and asteroids.

Staying on two legs is hard. Even people sometimes fall over. But animals with lots of legs don’t. So, scientists study insects (especially cockroaches, which for their size are the fastest land animal on earth) and spiders and even lobsters to imitate how they walk. This RoboLobster is designed to crawl underwater on the sandy bottom near the seashore.

Who needs legs? PaPeRo is a friendly little guy on wheels. When PaPeRo recognizes you, its eyes literally light up-they turn orange-and it turns its head to look at you when you talk. PaPeRo is designed to be a household companion. It can turn the lights on and off and change the TV channel on command. If it’s time to do your homework, your mom can email PaPeRo, and it will come to remind you. Although it won’t do your homework for you, PaPeRo will perform a little song and dance to cheer you up.

Meet Troody. She’s modeled after Troodon formosus, a carnivorous dinosaur of the Cretaceous period. It took five years for Troody’s inventor to get her to stand up from a sitting position without falling over, and then walk a few steps. If she were a real dino and that’s the best she could do, well, she’d be extinct.

This spider-bot has six legs and can fit in the palm of your hand. Scientists at NASA think that it might be better to send hundreds of little spider-bots, rather than one big rover, to crawl over the surface of planets scientists want to explore.

This bot is designed to slither like a snake, but it’s really rolling on wheels under its body sections.

This guard robot, Banryu, also has wheels on its legs. It roams your house, rolling smoothly when it can, then walking when the going gets tough. If it smells fire or senses a burglar, it calls you on its built-in cell phone.

Wheels and legs? This mechanical guy really “rolks.” On even ground, it rolls forward on its motorized wheels. But when the ground is loose or uneven, the robot rolls one wheel forward at a time-a combination of walking and rolling that its inventors call rolking.