Polyoxometalates (POMs) are useful in a wide range of biological applications and rare‐earth based POMs provide a potentially new biological optical label. The fact that the decay of lanthanide luminescence occurs on a timescaleslower than that of molecular fluorescence, typically micro‐ to milliseconds allows for discrimination from cellular autofluorescence. It also permits the probing of cellular processes that occur  on these longer timescales and not accessible via fluorescence techniques. The use of lanthanides is also advantageous as they are not as prone to photobleaching as organic dyes.