The underwater phase is a critical aspect of the swimming race where swimmers can gain a competitive advantage. A literature review revealed several areas lacking investigation, which would develop the body of knowledge surrounding underwater fly kick. Despite previous studies highlighting determinant factors of underwater fly kick performance, there is a lack of evidence available as to how to improve these aspects and coaches’ current practices not considered. This thesis aimed to assess the efficacy of short and long-term interventions in the development of underwater fly kick skills. The development of a three-dimensional motion capture methodology for the quantification of underwater fly kicking kinematics was detailed. Kinematics obtained from three and two-dimensional methods were compared, revealing differences in measured angles at every joint between 0.35 and 18.35 degrees. Correlation analysis indicated that out-of-plane motion, up to 36.69 degrees range of transverse plane and 19.26 degrees frontal plane motion and at the hip, affects the sagittal plane angle measured and is overlooked in two-dimensional analysis. A comparison of underwater fly kicking kinematics in eight male athletes (FINA points between 600 and 923) highlighted the importance of hip extension in maximising up-beat vertical toe velocity (r = .738, p = .037). Highly skilled athletes produced increased ankle only motion in coordination between transverse plane motion of the hip and ankle (r = .810, p = .015), and increased peak rotational velocity at the hip and ankle (r = .810, p = .015 and r = .738, p = .037 respectively). The current perceptions and practices of 56 coaches surrounding underwater fly kick development were explored. Coaches are considering complex concepts presented within the literature such as whole body coordination but lack the resource to implement best practices for skill development into training. The efficacy of common interventions were then assessed. The techniques of five elite male swimmers (FINA points: 884  41) in differing orientations were compared. A decreased up-beat duration (p = .042) indicated that supine swimming may facilitate increased kicking symmetry. However, this change in technique may be brought about through detrimental alterations; a lower kick frequency and reduced down-beat toe velocity. Two short-term interventions were then assessed in skilled male athletes (FINA points: 697  81); vertical kicking and resisted training. Vertical kicking improved kick symmetry, but the 4 % increase in up-beat duration diminished post across post intervention trials (p = .011). A resisted training intervention produced a 6 % increased swimming velocity post intervention (p = .019), but may have negative consequences upon technique through reduced hip motion and increased oscillation at the trunk and shoulder. Finally, the effects of a four-week skill acquisition intervention were assessed in a group of skilled male swimmers (FINA points: 681  62). Initially focussing on exploration of movement patterns, the intervention developed towards stabilising performance for competition. This intervention elicited improved performance through increased average and peak swimming velocity (p = .038 and .019 respectively) and decreased up-beat duration and symmetry between up and down-beat toe velocities (p = .022 and p = .002 respectively), enabling longer term development of techniques. Work packages within this thesis provide insights into the efficacy of current training practices, providing evidence for longer-term structured implementation of skill training theory driven practices in underwater fly kick coaching.