In this chapter, we apply the ideas and techniques developed in Chapter 3 to a class of nonnegative matrices arising in the modelling of population growth. We have already had a glimpse of this approach in section 1.1, where the Perron value and vector were seen to carry key information about the long-term behaviour of the Leslie population model. We begin by introducing the models in section 4.1, and then use the machinery of Chapter 3 to analyse the sensitivities of the Perron value and vector in sections 4.2 and 4.3. The related notion of elasticity is addressed in section 4.4.