This volume contains the proceedings of the 20th International Conference on AutomatedDeduction (CADE-20).ItwasheldJuly22-27,2005inTallinn,Es- nia,togetherwiththeWorkshoponConstraintsinFormalVeri?cation(CFV'05), the Workshop on Empirically Successful Classical Automated Reasoning (ES- CAR), the Workshop on Non-Theorems, Non-Validity, Non-Provability (DIS- PROVING), and the yearly CADE ATP System Competition (CASC). CADE is the major forum for the presentation of research in all aspects of automated deduction. The ?rst CADE conference was held in 1974. Early CADEs were mostly biennial, and annual conferences started in 1996. Logics of interest include propositional, ?rst-order, equational, higher-order, classical, intuitionistic, constructive, modal, temporal, many-valued, substr- tural, description, and meta-logics, logical frameworks, type theory and set t- ory. Methods of interest include saturation, resolution, tableaux, sequent calculi, term rewriting, induction, uni?cation, constraint solving, decision procedures, model generation,model checking,natural deduction, proofplanning, proof p- sentation, proof checking, and explanation. Applications of interest include hardwareand softwaredevelopment,systems analysisandveri?cation,deductivedatabases,functionalandlogicprogramming, computer mathematics, natural language processing, computational linguistics, robotics, planning, knowledge representation, and other areas of AI. This year, there were 78 submissions, of which 9 system descriptions. Each submissionwasassignedto atleastfour programcommitteemembers,whoca- fully reviewed the papers, in many cases with the help of one or more of a total number of 115 external referees. For each submission at least four reviews were produced and forwarded to the authors. The merits of the submissions were d- cussed by the programcommittee for ten days through the Internet by means of the EasyChair system. Finally, the program committee selected for publication 25 regular research papers and 5 system descriptions.