There is substantial evidence to suggest that parenting plays a significant role in children’s development. The Welsh Government (WG) has recognised that early intervention programmes, designed to enhance parenting skills, can address social inequalities and reduce poor child outcomes. The Incredible Years (IY) series has strong evidence for improving parent and child outcomes with children from three to eight years. Consequently, the Incredible Years Parent-Toddler Programme (IYPTP) was one of five parent programmes selected for use in Flying Start areas in Wales in 2007 as part of the WG Flying Start Initiative. The take up of the IYPTP by a significant number of Authorities provided the first opportunity to assess the effectiveness of the 12-week programme in a rigorous randomised controlled trial (RCT). Videotaped observations of parents and children interacting during free-play were collected during the main trial and consequently the thesis is comprised of a number of sub-studies resulting from this data. The aims and specific objectives of the thesis were to: 1. Identify the strength of the relationship between socioeconomic disadvantage and poor parenting behaviours for a sample of parents living in targeted Flying Start areas. 2. Develop a reliable observational tool to measure parental language in Wales. 3. Evaluate the efficacy of using a behaviour based parenting programmes to enhance parental language. The observational data provided seven categories of parental language for assessment, five complex and two simple. Socioeconomic disadvantage was strongly associated with reductions in simple quantifications of parental speech and home stimulation measures. Two or more risks indicated a threshold for poor outcome. Simple measures of parental speech were also fairly stable over time and moderately associated with child language outcomes six months later. In contrast, complex measures were less stable over the short-term, and relations with socioeconomic disadvantage and child language outcomes were weak. Results generated from the efficacy study indicated that the IYPTP, a behaviour based parent programme, provides some benefit to two complex measures of parental language six-months post intervention for a sample of parents living in disadvantaged areas in Wales. Whilst these findings should be considered in line with the methodological limitations of the study the thesis does conclude that parenting programmes designed to enhance parenting behaviours may not be as beneficial to parental language than those that contain a stronger component of language coaching.