Several iron-sensitive fluorophores have been investigated in a range of cell types in order to quantify iron(II) levels in the cytosol and the cytoplasm. Both iron(II) and iron(III) cause fluorescence quenching of these probes and changes in cytosolic iron levels can be monitored in a reproducible manner. However the precise quantification of iron(II) in the cytosol is complicated by the uncertainty of the structure of many of the quenched species that exist under in vivo conditions. Precise knowledge of these structures is essential for quantitative purposes. The lysosomal and mitochondrial iron pools have only been the subject of relatively few studies at the time of writing. Calcein–AM has been widely adopted for the monitoring of changes in iron levels in a range different cell types.