Wecompare13differentaposteriorierrorestimatorsforthePoissonproblemwithlowest-orderfiniteelementdiscretization.Residual-basederrorestimatorscompetewithawiderangeofaveragingestimatorsandestimatorsbasedonlocalproblems.Amongourfivebenchmarkproblemswealsolookontwoexampleswithdiscontinuousisotropicdiffusionandtheirimpactontheperformanceoftheestimators.(SupportedbyDFGResearchCenterMATHEON.)