What are the key concepts of functional programming?

Core concepts: 1) Pure functions - same input always gives same output, no side effects. 2) Immutability - data never changes, create new values. 3) First-class functions - functions as values, passed and returned. 4) Higher-order functions - take or return functions (map, filter, reduce). 5) Referential transparency - expression replaceable by its value. 6) Recursion instead of loops. Benefits: predictable, testable, parallelizable. Languages: Haskell, Clojure, Scala, F#. FP features in mainstream: lambdas, streams (Java), comprehensions (Python), array methods (JavaScript).