Neonatal circulatory adaptation at birth is unique. If the transition from an in- to ex-utero circulatory system is unsuccessful, circulatory failure ensues resulting in anaerobic respiration and eventual tissue death. This thesis explores the use of novel non-invasive techniques to assess neonatal circulatory adaptation. Data are reported from three observational cohort studies including infants aged less than 72 hours of age and greater than 33 weeks’ gestational age (GA) who received special care (n=50), intensive care (n=25) or total body cooling (n=14). For the first three days of life infants had routine daily clinical assessments (e.g. blood pressure), echocardiographic (superior vena cava flow and right ventricular outflow) and plethysmographic measurements (modified pleth variability index and pulse transit time). Daily longitudinal, comparative and correlational analysis of these measures within and between cohorts of neonates were performed. In addition, their relationships to assessments of short term neurological outcomes and cardiovascular treatment were explored. Bland Altman plots were used to explore the repeatability of plethysmographic and echocardiographic measures. The results indicate that the cardiovascular systems of between the three cohorts of neonates studied adapt differently over the first three days of life. Specifically, neonates who receive total body cooling exhibited significantly lower blood pressures, heart rates and measures systemic blood flow compared to neonates who are healthy or receiving intensive care. Healthy neonates aged between 33 to < 37 weeks GA exhibited daily significant shortening of modified pulse transit time and increased measures of systemic blood flow indicating these neonate’s systemic vascular resistances increase more gradually compared to term neonates with end organ perfusion maintained through increased cardiac output. The intra- and inter-observer repeatability of echocardiographic and plethysmographic assessments was poor and excellent respectively (repeatability index range 26-64% vs. 3-13%). Future studies should focus on the use of these biomarkers in the identification of neonates at risk of circulatory failure.