This thesis is concerned with the dynamics of the health of populations with low levels of mortality, using the non-Maori population in New Zealand as a case study. The primary objective is to study patterns and trends in mortality and morbidity, their interactions, and their implications for the future. The key questions addressed in this thesis are central to the future evolution of population health. Given the significant declines in mortality, especially those at older ages that have occurred over the last two decades, will mortality declines at advanced ages continue much longer? Or will mortality become increasingly compressed into a narrow range of ages at death? Moreover, what are the implications for morbidity trends and any indications at present of future trends in population health? Will increased survivorship be accompanied by long periods in ill-health, or will morbidity move in tandem with mortality, or will durations surviving but in ill-health actually decrease? The thesis first establishes a theoretical platform by reviewing and synthesising transition theories and empirical observations into a general framework to guide the understanding of population health dynamics. It also forges a theoretical linkage between variations in the individual senescence process, which underpin the validity of demographic models of mortality and morbidity, and their population level manifestations. These theoretical extensions provide the context for an evaluation of contemporary theories on mortality trends and changes in morbidity in relation to mortality. In addition to using long term historical data spanning over a century and complemented by a population-based true cohort approach, this thesis also makes methodological advancements by adapting and developing a tool, known as health expectancy, for the analysis of population health. In particular, a new measure termed “hospitalisation expectancy” is developed. It integrates data on hospital utilisation and mortality to yield a summary measure of population health. The empirical study documents the shift in the forces of mortality of non-Maori, from younger to older ages, and from communicable causes of death to chronic degenerative diseases. Results of the mortality analysis by cohorts show that while there may be some indications of a future trend towards mortality compression, further increases in life expectancy at birth can be realistically expected. Empirical evidence of compression of morbidity is found in the trends of hospitalisation and the patterns of severe forms of disability. The causes of morbidity are becoming more diversified, dominated by non-lethal conditions; whereas the causes of mortality are becoming increasingly compressed into a few chronic conditions, namely cancers and cardiovascular diseases. It is also established in this thesis that the older population represents a heterogeneous group in terms of levels of susceptibility to ill-health. An empirical method for identifying heterogeneity is developed to show non-Maori inter-cohort differences in this property. Later in the thesis, a cohort based mortality projection of non-Maori at adult ages is undertaken. Various scenarios of morbidity responses to the projected mortality changes, incorporating the leading theories on future morbidity changes, are then modelled. To conclude the thesis, the implications for policy of further mortality declines at older age among an increasingly heterogeneous population are discussed.