Metapragmatic awareness (MPA) is the ability to explicitly reflect upon the pragmatic rules that govern conversation. There is a paucity of research on how MPA develops in childhood and whether it is impaired in children with pragmatic impairments. Despite this, MPA is often cited as an intervention tool for children with pragmatic language impairments (cwPLI) and children with specific language impairments (cwSLI). There are currently no published assessments of MPA ability and practice would benefit from application of a formalised assessment methodology. This thesis reports the phases of development of a novel clinical assessment of MPA for school-aged children called the Assessment of Metapragmatics (AMP). The AMP task is a set of 13 Video Items each depicting a conversation between pairs of school-aged children. Each Video Item portrays a different pragmatic rule violation. After viewing each AMP Video Item the participants were asked a set of Assessor Questions designed to measure MPA. The AMP Video Items were shown to 40 children with typical language development (cwTLD), 34 cwPLI and 14 cwSLI. Preliminary analyses revealed the AMP to be sensitive to age-related changes in MPA and to demonstrate good internal reliability. For the cwTLD there was a distinct developmental shift in MPA ability around seven years of age. At this age there was an increase in the child’s ability to use explicit metapragmatic vocabulary to describe a pragmatic rule violation. CwTLD demonstrated superior MPA ability in comparison to the cwPLI and the cwSLI. No differential impairment in MPA abilities was present between the cwPLI and cwSLI. Considerable variability in MPA abilities occurred for both the cwPLI and cwSLI and this was associated with language ability. This suggests that where MPA is found to be impaired, the child’s language ability should be taken into account and that language ability should be remediated before MPA is targeted in intervention. Where MPA is impaired, raising awareness of pragmatic rule may be the first step for intervention. Where MPA is age-appropriate, the child’s ability to monitor their use of the pragmatic rule, or their motivation to use the pragmatic rule, may be a more effective target of intervention in order to change behaviour. The relationship between MPA and social understanding for the pragmatic rule violation is also discussed and further studies of MPA are considered.