Clojure has an immutable data model. However, this doesn't mean that we can't represent data and change it over time. Direct change or mutation should be avoided at all costs. If necessary, a binding macro will create new var values (or bindings) that are local only to a binding context or scope. These Vars must be marked as ^:dynamic and they should already exist:

(def ^:dynamic a 1)
(def ^:dynamic b 2)

(binding [a 10 b 20] (+ a b))
; => 30

a
; => 1

b
; => 2

It is, however, preferable to choose one of four reference types: Var, Atom, Agent, and Ref. Doing this requires an understanding of Clojure's notion of identity, value, and time. In his presentation Are We There Yet? (you can read more at http://www.infoq.com/presentations/Are-We-There-Yet-Rich-Hickey), Clojure creator, Rich Hickey, describes most computer languages as having variables that allow programmers to set and change a value in the program's runtime. This model assumes, however, that the world stops while you look at or change it (you can read more at http://clojure.org/state). Clojure's approach includes having an immutable value, such as Sally's address, and forever maintaining this value at a point in time. Sally's address, however, is just a value in 2014. Clojure sees identity as a continuous logical entity that has a series of different values over time. So, Sally's home, would be an identity where the value of her address in 2014 may be different from her address in 2015. As we've seen, Clojure's core functions take immutable values and return new values that are updates of the original. The two values share data structures under the hood (Clojure's Persistent Data Structures), which is possible since each value won't change. This succession of data updates is how we disambiguate between values over time. These values often need to be shared with other threads of execution. The Var, Atom, Agent, and Ref reference types are ways of managing shared identities that have a particular state at a given point in time. These reference types can change using a prescribed set of functions to create, update, and reset their state: