The influence of the k'th coordinate on a Boolean function f:{0,1}^n -> {0,1} is the probability that flipping x_k changes the value f(x). The total influence I(f) is the sum of influences of the coordinates. The classical Junta Theorem' of Friedgut (1998) asserts that if I(f) <= M, then f can be \epsilon-approximated by a function that depends on O(2^{M/\epsilon}) coordinates. Friedgut's theorem has a wide variety of applications in mathematics and theoretical computer science.