  In this paper, we discuss centralizers in the Riordan group. We will see that
Fa\`a di Bruno's formula is an application of the Fundamental Theorem of
Riordan arrays. Then the composition group of formal power series in ${\cal
F}_1$ is studied to construct the centralizers of Bell type and Lagrange type
Riordan arrays. Our tools are the $A$-sequences of Riordan arrays and Fa\`a di
Bruno's formula. Some combinatorial explanation and discussion about related
algebraic topics are also given.
