Exposure to long-chain saturated fatty acids (SFAs; e.g. palmitate) induces apoptosis in pancreatic β cells, a process that may contribute to the development of type 2 diabetes. Under palmitate treatment, β cells undergo a so-called endoplasmic reticulum (ER) stress that can be counteracted by the unfolded protein response (UPR). The UPR is a coordinated response, which is primarily devoted to helping the ER to cope with the accumulation of misfolded proteins. Sustained SFA exposure may ultimately overwhelm the UPR, resulting in cell death. By contrast, unsaturated fatty acids (e.g. oleate) are much less harmful to the cells and can even alleviate palmitate toxicity. Surprisingly, recent evidences indicate that a simple unicellular eukaryote, the budding yeast Saccharomyces cerevisiae, which is not routinely exposed to high-fat diets, also undergoes ER stress under lipotoxic conditions. This suggests that the mechanisms of SFA toxicity are largely conserved throughout eukaryotes and are not specific of a given cell type. The present review discusses the mechanisms of SFA toxicity in yeast and β cells, with a main emphasis on their potential impacts on ER-membrane organization/function and ER-based processes.