In this chapter we introduce the notion of parameterized reductions. We explain how this technique can be used to transform an input for a parameterized problem $K$-$A$ into an input or parameterized problem $K$-$B$, mapping yes-instances for $K$-$A$ to yes-instances for $K$-$B$ and vice versa. If this transformation can be done in fixed-parameter tractable time, this implies that if $K$-$B$ is fixed-parameter-tractable, then so is $K$-$A$; conversely, if $K$-$A$ is not fixed-parameter tractable, then neither is $K$-$B$. Like the polynomial-time reductions introduced in Chapter 3, parameterized reductions are a powerful technique for relating problems to each other. We will demonstrate parameterized analogues of each of the reduction strategies described in Chapter 3. We also include several exercises for practicing this technique.