This action research explores the effects of formative assessment on teaching and learning. Prior to embarking on this research, the researcher had relied on summative methods of assessing the attainments of pupils in her class. The values underpinning this research and which have led to this study are: equality, inclusion, perseverance and care. The research question may be framed thus: what impact has the implementation of formative assessment methodologies on children’s learning and on a practitioner’s efficacy as a teacher? The research was conducted in a co-educational, Catholic Primary School in Co. Kildare. The research participants were the children in the researcher’s Third Class. Ten target pupils were selected using a random, stratified sampling method for the duration of the study. The methodology employed was a mixed-methods qualitative and quantitative approach. It consisted of two cycles which lasted six weeks in total. Cycle One explored the introduction of a growth mindset environment into the researcher’s classroom, as the literature confirmed that this was a highly advantageous prerequisite to using formative assessment. Various lesson plans and interactive games which helped widen the children’s vocabulary and open their minds were used. A classroom display about changing mindsets was co-created. Cycle Two explored the implementing of formative assessment methodologies. These included The Jigsaw Strategy, Determining Success Criteria, Peer Marking and Reflective Journaling. The formative assessment methodologies were introduced throughout the school day during various subjects including English, history, geography and science. Six research instruments were used as a means of collecting data. They included pre- and post-mindset worksheets, a post-research questionnaire, children’s work samples, critical friends’ contributions, a personal reflective journal and teacher observations. As a result of carrying out this research, seven main findings emerged. These may be subdivided into two categories: learning and teaching. In terms of learning, the first finding was that children’s mindsets, their self-esteem and their attitude towards learning, were affected in a positive way. Secondly, children enjoyed and were capable of giving feedback and benefitted from being allowed to reflect upon their work. Thirdly, there was an improvement in the quality of the children’s work, particularly when the success criteria was co-created and the learning intention shared. In terms of teaching, the first finding was that it is important to create a learning environment that is collaborative. Secondly, the employment of formative assessment in the classroom had a positive effect on the researcher’s feelings of efficacy and integrity as a teacher. Thirdly, the importance of tapping into children’s preoccupations and interests and keeping learning fun emerged. Fourthly, a willingness to engage in self-reflection provoked a change in the researcher’s personal teaching practice. There are many benefits to complementing teacher assessment with formative assessment as in the case of the four methodologies chosen for this research. This study has highlighted the importance of promoting a growth mindset on the part of both the teacher and the children.