This issue of Essays in Biochemistry deals with lipid mediators, which are an exciting and ever-expanding series of biological molecules. Perhaps the first well-studied of such mediators were the inositol lipids whose rapid turnover was noted over 50 years ago. Since then, a huge variety of lipids and their metabolites have been characterised as lipid mediators, with many important functions in different eukaryotic organisms. In this volume we have tried to include as many notable examples as we could. The individual mediators include complex lipids (known as membrane components), intermediates in metabolism, as well as oxidised products of polyunsaturated fatty acid catabolism. The properties of many of these compounds are still being elucidated but offer important opportunities for exploitation in agriculture, industry and medicine. While the volume was being compiled, Mike Wakelam (a good friend and colleague of lipid scientists) unfortunately died prematurely from the effects of Covid-19 infection. It is our honour to dedicate this issue of Essays in Biochemistry to his memory. The cover image shows, clockwise from top left: Caterpillars eating a leaf, where jasmonic acid is secreted in defence ("Larval lepidopterans on buttonbush leaf [Newark, Ohio, USA] 1" by James St. John is licensed with CC BY 2.0); Zebrafish developing brain, showing presence of steroids (associated structure is the neurosteroid allopregnanolone), brain stained with filipin by Dr Emyr Lloyd-Evans, School of Biosciences, Cardiff University; Mouse embryo vasculature stained with PECAM, organised by S1P signalling (which is the lipid structure associated with the image) by Dr Helen Waller-Evans, Medicines Discovery Institute, Cardiff University; Healthy brain MRI with Protectin D1 (PD1) ("MRI" by jillccarlson is licensed with CC BY 2.0).