This chapter offers an overview of psychological developmental theory and its application to children’s nursing practice. Particular emphasis is placed on cognitive development, and the work of Jean Piaget, Lev Vygotsky and other contributors to the field is critically appraised. Implications for children’s nursing practice are articulated by exploring the application of the different psychological developmental theories to children’s understanding of health and illness concepts. The chapter then considers language development and personality development, again exploring the implications of these theories within children’s nursing