To improve learning-outcomes teaching quality matters. However, research into teaching in low- and middle-income countries (L&MIC) is limited, particularly in mathematics and the rapidly rising low-cost private sector (LCPS). The purpose of this research is to study mathematics teaching and learning in Karachi's LCPS by exploring four related aspects: who attends Karachi's LCPS-school; the values that underpin LCPS teachers' instructional and professional practices; the instructional practices teachers use to teach mathematics; and the institutional environment that supports or hinders the development of mathematics teaching practice. Five key components of effective teaching underpinned by communicative pedagogies are explored in this study in relation to LCPS teachers' practice: planning and preparation; a conducive classroom environment; effective instructional practices; independent practice and summative assessment; and teachers' role as professionals. Employing a mixed-method case-study approach, this study uses primary data gathered through lesson observations and interviews conducted in five pilot and two in-depth case-study LCPS-schools, and secondary quantitative data. This thesis employs a pragmatic perspective on the school effectiveness and improvement research framework and argues for its greater use in identifying good practice in L&MIC. LCPS teachers are unqualified, untrained and poorly paid but driven by a strong sense of moral purpose underpinned by a transformational view of education. They exhibit a continuum of practice from novice to expert with the latter reflecting the same components of effective practices found in HIC literature. My findings show LCPS teachers can be supported to become expert through a systematic programme of professional development and a supportive accountability framework. Therefore, this study argues for support to be provided to LCPS teachers on developing students' conceptual understanding, embedding formative assessment and promoting mathematical communication. It concludes with recommendations for policymakers to engage with the LCPS at a systemic level to promote equity and improve learning.