Physical activity (PA) recommendations in the United Kingdom (UK) state that children aged 5 and under should be active for at least 180 minutes each day, with at least 60 from moderate to vigorous PA (MVPA). Good motor development aids children in participating in adequate levels of PA, and during the early years of childhood (3-5 years) children should begin to learn, practice, and develop fundamental movement skills (FMS). With declining childhood PA levels in the UK and a growing gap in PA performance due to deprivation, more must be understood about which key factors affect children's FMS development and PA participation. Crucially, the appropriate support and intervention to improve these factors must be informed and underpinned by a theoretical and evidence base from this population and their environment. The aims of this thesis were to a) systematically review the literature available about 4-5 year old's FMS, and accelerometer measured PA levels, including the variation in measurement methods employed, b) investigate educators perspectives of PA, FMS and physical education (PE) for early years foundation stage (EYFS) children, c) assess a sample of English EYFS children's FMS competency and PA, finally, d) be the first to use intervention mapping (IM) to develop appropriate interventions to aid FMS development and PA participation in early childhood in school settings informed by the data sets collected through this programme of research. The initial study of this PhD systematically reviewed the literature available for children aged 4-5 years of age's FMS levels and accelerometer-based PA levels. Following PRISMA guidelines 3 data bases were searched for appropriate articles before being screened for inclusion. Results showed that children's FMS competency levels were lower than expected for their age, PA was generally underperformed at MVPA levels, and that stability measurements were wide and varied for this age group. The review also focussed on the measurement methods and issues within the research area, with suggestion on how some of these issues should be overcome or improved. The second study of the programme collected the perceptions that EYFS educators in England held about PA, FMS, and PE for children. Twelve educators (two headteachers, three external coaches, and seven EYFS teachers) took part in semi-structured individual interviews. Educators were from locations across England and taught in a range of SES schools and were predominantly female (n=9). Thematic analysis was employed to identify key themes formed from codes within verbatim transcription of the interviews. The five emergent themes were: PA and PE in the EYFS; the benefits of PA and PE for young children; the barriers and challenges to achieving sufficient PA/PE for children faced by educators, and children; educator knowledge of FMS and key opportunities for development; intervention experience, needs, and training delivery. The third study assessed the FMS competency, including balance (n=92), and accelerometer measured PA levels (n=54) of a sample of children from the central region of England attending the EYFS of school. This study aimed to understand the current achievement of English children and establish if a relationship between FMS and PA exists, and what variables may affect this. The key analysis showed that age tertiles of a school year (autumn, spring, and summer term births) had an effect on the children's FMS competency level, with the oldest children achieving the best level of FMS competency. The participants with sufficient PA data showed that they were meeting PA guidelines of both total PA (180 minutes) and MVPA (60 minutes) according to British population cut points for wrist worn accelerometers. This study also was adapted for an online data collection during the coronavirus pandemic, this resulted in a very small sample of data collection, although this was not used in the synthesis of results. The important outcomes from this study included the challenges that are faced during remote data collection and recommendations of how to best collect data in these circumstances. This adds key knowledge to this area of research, particularly from a methodological perspective. The final study was the most novel in nature, employing IM, a six-stage process used to design and develop programmes of theory and evidence-based health promotion interventions. To our knowledge this study was the first to begin to use IM to plan FMS promotion interventions and particularly focuses on early childhood in English school settings. By using the data sets collected throughout this programme of research, in addition to a planning group of key stakeholders, each task of the six steps of IM were detailed, including the planning, and drafting of selected programme materials. This begins to guide the development of an EYFS school-based intervention to implement both at the school setting, local authority, and community setting, with consideration to the developments needed at policy level intervention. The studies in this thesis extend the knowledge within the area of PA and FMS in early childhood populations and specifically of those children at the start of school within English populations. The thesis has found that globally the literature reports FMS competency to be low in children aged 4-5 years and there is varying evidence surrounding performance PA levels, with a lack of consensus on how to best measure FMS and accelerometer measured PA in early childhood. This low global FMS competency is supported by the primary observations of 4-5-year-olds in central England, however these children did demonstrate sufficient PA levels, adding to the varying evidence available. Despite this, in England, educators reported that they value the use of PE and PA opportunities to help children develop physically, mentally, socially, and academically. Nonetheless, a lack of confidence in knowledge of FMS and application of how to teach and create opportunity leaves some teachers unable to promote and teach better FMS competency. Importantly, this thesis has developed a programme of FMS promotion interventions to be used in school settings and provided at teacher (implementer) and child (individual/participant) level. By considering the different levels of the socioecological model, the adaptability to meet different school's needs across England can be achieved, which meets the aims of the IM process.