DEMO - a demo of epistemic modelling. This paper introduces and documents DEMO, a Dynamic Epistemic Modelling tool. DEMO allows modelling epistemic updates, graphical display of update results, graphical display of action models, formula evaluation in epistemic models, translation of dynamic epistemic formulas to PDL formulas. Also, DEMO implements the reduction of dynamic epistemic logic to PDL. The paper gives an exemplar of tool building for epistemic update logic. It contains the essential code of an implementation of DEMO in Haskell, in Knuth’s ‘literate programming’ style.