The level of maturity that has been reached by model transformation technologies is proved by the growing literature on transformation libraries that address an increasingly wide spectrum of applications. With the success of the modeling and transformation paradigm, the need arises to address more complex applications that require a direct manipulation of model transformations. The uniformity and flexibility of the model-driven paradigm allows this class of applications to make use of the same transformation infrastructure. This is possible because transformations can be translated into transformation models and given as objects to a different class of model transformations, called Higher-Order Transformations (HOT). This paper provides an introduction to HOTs and a survey of the several application cases where their use is relevant. A number of possible future applications of HOTs is also proposed.