**1. Introduction**

In this chapter we will explore the current understanding about the dynamics of myocardial cell populations after birth and how they may contribute to homeostasis and response to injury. Recent investigations of myocardial cell biology have revealed that the heart is not the terminally differentiated organ it was once thought to be. We now know that myocardium contains limited populations of cardiac progenitor cells (CPC) that are capable of generating all myocardial cell types. The changes in myocardial cell populations with time and the role these changes play in neonatal myocardial tissue expansion will be discussed. The contribution of CPC to heart growth and therapeutic strategies for congenital cardiac diseases will be explored.
