Most, but not all monkeys have opposable thumbs. Old world monkeys, humans and apes have opposable thumbs, as do pandas and opossums.
An animal is said to have an opposable thumb if the thumb can be bent in a way that it can touch all the other fingers and toes on the same hand or foot. Generally, only primates have opposable thumbs but it is seen in other animals.
Primates that do not have use of an opposable thumb include the marmoset and the tarsier. Many monkeys, such as spider monkeys, have a thumb, but it is not opposable. The colobus has thumbs that do not move in the same way as an opposable thumb. Primates lacking this appendage often compensate by using their tails to aid in locomotion and other daily tasks.