Although lipids are biomolecules with seemingly simple chemical structures, the molecular composition of the cellular lipidome is complex and, currently, poorly understood. The exact mechanisms of how compositional complexity affects cell homeostasis and its regulation also remain unclear. This emerging field is developing sensitive mass spectrometry technologies for the quantitative characterization of the lipidome. Here, we argue that lipidomics will become an essential tool kit in cell and developmental biology, molecular medicine and nutrition.