In this section, we take a step away from transactional references, and study more powerful transactional constructs, namely, transactional collections. While transactional references can only hold a single value at once, transactional collections can manipulate multiple values. In principle, the atomic statements and transactional references are sufficient to express any kind of transaction over shared data. However, ScalaSTM's transactional collections are deeply integrated with the STM. They can be used to express shared data operations more conveniently and execute the transactions more efficiently.