Extremely low gestational age infants (<28 weeks at birth) experience significant pain from repeated therapeutic procedures while hospitalized in the neonatal intensive care unit. As part of a program of research examining pain in preterm infants, we conducted a qualitatively driven mixed-methods design, supplemented with a qualitative and quantitative component, to understand how health care professionals (HCPs) assess and manage procedural pain for tiny and underdeveloped preterm infants. Fifty-nine HCPs from different disciplines across four tertiary-level neonatal units in Canada participated in individual or focus group interviews and completed a brief questionnaire. Four themes from the content analysis were (a) subtlety and unpredictability of pain indicators, (b) infant and caregiver attributes and contextual factors that influence pain response and practices, (c) the complex nature of pain assessment, and (d) uncertainty in the management of pain. The information gleaned from this study can assist in identifying gaps in knowledge and informing unit-based and organizational knowledge translation strategies for this vulnerable population.