Summary Infants born very preterm - before 32 weeks gestation and ≤1500 grams birthweight - are at risk of developmental problems. Gross motor development is an early marker in the first year of life to identify developmental delay. As in other developmental domains, gross motor development is characterized by variability between infants known to be related to both infant and environmental factors. This variability makes predicting infant motor development difficult and with that clinical decision-making on initiating early intervention. Repeatedly, within the same child, measuring gross motor development over time makes discrimination of typical or atypical development and developmental progress possible. The Alberta Infant Motor Scale (AIMS) is an observational instrument frequently used by paediatric physiotherapists worldwide to measure infant gross motor development. Among other things, this dissertation describes cross-cultural research to examine the validity of the Canadian norm values of the AIMS for the Dutch population of infants from birth to independent walking. Next, a longitudinal study was conducted to explore the variation in course and speed of gross motor development of Dutch very preterm infants until independent walking and influencing factors. Parents made multiple home video recordings of their infant (corrected age 3.5 to 17.5 months), from which gross motor development was assessed with AIMS. Lastly, parental beliefs about their child's motor development and experiences with the home-video method were examined. Main outcomes of the studies are 1) Dutch infants develop at a slower pace than Canadian infants from birth until independent walking, 2) there is strong evidence for the association of birthweight and moderate evidence for gestational age and sleeping position with infant gross motor development, 3) gross motor growth curves showed that three profiles, comparable to profiles observed among term born infants, can be distinguished: early developers, gradual developers and late bloomers and, 4) parents of very preterm infants believed stimulating motor development to be more important than parents of term born infants.