BACKGROUND: Assessment of health facility service readiness for small and sick newborn care (SSNC) can help identify health systems gaps, which need to be addressed, to prevent the estimated 2.3 million annual newborns deaths. However, existing tools did not assess service readiness for SSNC. Further, there were no standardised approaches for quantitatively evaluating service readiness for SSNC to enable comparison between facilities and tracking over time. This PhD was embedded within the NEST360 Alliance, an alliance aiming to reduce newborn mortality by co-creating and implementing a health systems package with governments in Kenya, Malawi, Nigeria, and Tanzania. METHODS: The aim of this PhD was to advance measurement of standards-based service readiness for level-2 inpatient SSNC in low- and middle-income countries (LMICs). The PhD covers four objectives: 1) to develop a service readiness tool; 2) to develop and apply two scoring approaches; 3) to evaluate incremental service readiness change; and 4) to analyse trends in staff-to-baby ratios and health workforce readiness. A new Health Facility Assessment (HFA) tool for SSNC in LMICs was systematically developed. Using data from 64 hospitals in Kenya, Malawi, Nigeria, and Tanzania, two scoring approaches were developed to aggregate and summarise the HFA data; (a) standards-based scores by health system building blocks (HSBBs), and (b) World Health Organization (WHO) level-2+ scores by clinical intervention. Then, scores from these two approaches were evaluated at baseline and follow-up (~31 months) to understand incremental change in service readiness for SSNC. Finally, health workforce readiness, including trends in staff-to-baby ratios, was evaluated over time to understand human resourcing and how it might impact SSNC. RESULTS: Existing HFA tools only covered a third of health systems items necessary for SSNC according to international guidelines. Informed by these gaps, a new tool was co-developed systematically with >150 global experts, including government representatives, United Nations Children's Fund (UNICEF), and the NEST360 Alliance. The two scoring approaches developed showed low hospital readiness for SSNC at baseline, which varied by HSBBs with a median score of 51% [IQR 48-57%] and by clinical intervention with a median score of 41% [IQR 35-51%]. However, readiness for SSNC improved over time. The median percent score change between baseline and follow-up was 9% [IQR 3-11%] for standards-based scores and 14% [IQR 4-18%] for level-2+ scores with variations by country. Despite improvements in health workforce readiness, between baseline and follow-up, median nurse-to-baby ratios were 1 to 6 (IQR 1 to 3 – 1 to 11) during the day and 1 to 10 (IQR 1 to 6 – 1 to 16) at night, and many hospitals did not have doctors providing 24-hour coverage. DISCUSSION: The findings of this PhD, particularly the novel SSNC service readiness tool and methods for quantifying SSNC service readiness data, can support hospitals and governments to identify gaps in the care of small and sick newborns. This work could inform resource allocation and accelerate efforts towards improving newborn survival in LMICs to meet sustainable development goals.