The following is Part 1 of a 2 part series on development. Part 1 focuses on physical and cognitive development in humans; we will begin by exploring three basic issues. Firstly, we will discover the extent that the development process is the product of either nature or nature. Secondly we will discuss evidence for critical periods in psychological development. And finally, we will consider whether development occurs in stages or in a continuous, gradual change. After all of this, I will outline the most important psychological model of development that we have in cognitive psychology – Piaget’s theory of cognitive development.