Chapter 2 of the Health Effects of Climate Change (HECC) in the UK: 2023 report. This chapter presents new empirical analyses to estimate the likely impacts of changing climate on future temperature-related mortality in the UK. This work was conducted by the UKHSA in partnership with academic experts in temperature-health modelling at the London School of Hygiene and Tropical Medicine, University of St Andrews, University of Oxford, University College London and Australia National University. These new analyses use updated climate projections and epidemiological methods to estimate mortality associated with heat and cold across the UK up to the 2070s.