The Trends in International Mathematics and Science Study (TIMSS) is an international comparison study of mathematics and science performance, organised by the International Association for the Evaluation of Educational Achievement (IEA). The study's main purpose is to provide participating countries with internationally comparable data on the performance and attitudes of 9 to 10 year olds (year 5 in England) and 13 to 14 year olds (year 9 in England) in mathematics and science, together with comparisons of the curriculum and the teaching of these subjects in primary and secondary schools. Sixty-six countries and 6 benchmarking systems participated in TIMSS 2023. England has participated in every TIMSS since the study was first carried out in 1995, and the results provide valuable information on trends in England's absolute and relative performance. This report focuses on changes over the last 20 years (2003 to 2023) since in 1995 the TIMSS sample comprised both year 4 and 5 pupils and in 1999 no year 5 pupils participated. In addition, the timing of the assessments were later from 2003, giving pupils more learning time prior to taking the tests. In England, testing was conducted with pupils in years 5 and 9 between March and June 2023, with a sample of 8330 pupils across 267 schools. All pupils took the tests online. England's year 5 cohort started school in 2017. These pupils were at the end of Key Stage 1 in Summer 2020, but did not take part in statutory assessments at that point because of the COVID-19 pandemic. The year 9 cohort started primary school in 2013 and secondary school in 2020, and will take their GCSEs in summer 2025. They did not take statutory external assessments at the end of their year 6 because of the pandemic. This "Trends in International Mathematics and Science Study (TIMSS) 2023: National Report for England Volume 1" focuses on comparisons of England's pupils' performance in mathematics and science with the performance of pupils in the highest-performing countries, other English-speaking countries and a selection of other European countries. "Trends in International Mathematics and Science Study (TIMSS) 2023: National Report for England Volume 2," published in March 2025, focuses on performance in England analysed by pupil characteristics and participating pupils' reported experiences of mathematics and science teaching and learning, including some international comparisons. The TIMSS International Report 2023 offers comparisons across all participating countries.