The relationship between plant form and climate has been recognised for more than two millennia, and the idea that fossils can indicate climate change was first recorded almost a thousand years ago. Here we review ways of using plant form to reconstruct, quantitatively, past climates, focussing on techniques that have been developed over the past century. We begin with the relationship between woody dicot leaf margins and temperature, and trace the development of the approach through to the modern multivariate tool known as Climate-Leaf Analysis Multivariate Program (CLAMP), which can derive 24 different climate parameters from 31 macroscopic leaf traits commonly preserved in leaf fossils. We review the complex inter-dependant correlations between leaf traits and climate, recognising that leaves develop and function as integrated systems, and that there is a selective premium on them being well adapted to their immediate climatic environment. We discuss different ways of understanding and decoding climate from leaf form in multidimensional trait space, and provide an over-view of CLAMP applications for tracking climate change, monsoon evolution, and in palaeoaltimetry, from the middle Cretaceous to the Pleistocene.