OBJECTIVE The present review summarizes recent findings that may help in understanding how the cell senses changes in serum phosphate. RESULTS The sensing of phosphate determines the organism's response to change in supply of this essential nutrient. Phosphate depletion or surfeit results in homeostatic responses that involve changes in transcription, transcript stability, transporter recruitment or breakdown, and cell replication. These responses are shared across the biological kingdoms, and lessons from unicellular organisms may be relevant to multicellular mammals. An understanding of nutrient sensing in general may help in determining how the cell senses changes in phosphate concentration. CONCLUSIONS Research has yielded important advances in unravelling phosphate sensing and the response to nutrient phosphate supply. However, the actual sensing event for phosphate and most other nutrients must still be defined. Lessons may be learned from those examples in which the sensing event is known, and these are summarized here.