Let's face it we need/want to know where our monkeys are at all times. We strongly recommend that your baby learn to wear some type of collar/harness device to keep them safe. It's an important part of bonding/training as well. We have used all kinds here at Monkeys *R* Us in order to keep our monkeys safe and know where they are at all times. We prefer these belly collars because they stay on. We have lots of monkeys and it can be a hassle putting harnesses on and off throughout the day. Your monkey will wear this soft collar right above their hips at the smallest part of their waist (under their belly). These belly collars are made paracord style using the softest Bernat Baby Soft Yarn to protect baby-adult delicate skin and hair. Soft and comfortable enough to sleep in. Comes in an assortment of colors. The tiny bell lets you know where they are at all times. These collars are not adjustable so it's important to replace as they grow. They will usually go through 3-4 sizes as they grow the first year. After that they usually stay the about the same size. However, just like us- belly sizes go up or down from time to time. I usually wash & dry belly collar when I give monkeys their bath. When ordering measure your monkey at the smallest part of their waist (under their belly) not tight/loose. Too tight is uncomfortable and too loose will not stay on. Sizes go up 1/2 inch each size. Babies usually go home with a 3 1/2 inch (newborn marmoset/tamarin size) which is usually a little big, but they are growing so fast. They usually wear it the first 2-3 months then measure to see if they need 4 inch etc. If you don't see the size you need just call me and I'll work to get just the right size for your critter. The Newborn Belly Collar comes with a matching Bernat Baby Soft lead. Leads for other sizes may vary. For Adult Monkeys I recommend the Paracord Lead. It's durable and tends not to wrap around cage items if left on for a bit in the cage. As with anything that goes on your monkey check for fit, comfort & safety.