This book constitutes the proceeding of the 27th International Conference on Automated Deduction, CADE 27, held in Natal, Brazil, in August 2019. The 27 full papers and 7 system descriptions presented were carefully reviewed and selected from 65 submissions.

Automated Reasoning for Security Protocols.- Computer Deduction and (Formal) Proofs in Mathematics.- From Counter-Model-based Quantifier Instantiation to Quantifier Elimination in SMT.- The CADE-27 ATP System Competition - CASC-27.- Unification modulo Lists with Reverse - Relation with Certain Word Equations.- On the Width of Regular Classes of Finite Structures.- Extending SMT solvers to Higher-Order Logic.- Superposition with Lambdas.- Restricted Combinatory Unification.- dLi: Definite Descriptions in Differential Dynamic Logic.- SPASS-SATT { A CDCL(LA) Solver.- GRUNGE: A Grand Unified ATP Challenge.- Model Completeness, Covers and Superposition.- A Tableaux Calculus for Default Intuitionistic Logic.- NIL: Learning Nonlinear Interpolants.- ENIGMA-NG: Efficient Neural and Gradient-Boosted Inference Guidance for E.- Towards Physical Hybrid Systems.- SCL -- Clause Learning from Simple Models.- Names are not just Sound and Smoke: Word Embeddings for Axiom Selection.- Computing Expected Runtimes for Constant Probability Programs.- Automatic Generation of Logical Models with AGES.- Automata Terms in a Lazy WSkS Decision Procedure.- Confluence by Critical Pair Analysis Revisited.- Composing Proof Terms.- Combining ProVerif and Automated Theorem Provers for Security Protocol Verification.- Towards Bit-Width-Independent Proofs in SMT Solvers.- On Invariant Synthesis for Parametric Systems.- The Aspect Calculus.- Uniform Substitution At One Fell Swoop.- A Formally Verified Abstract Account of Gödel's Incompleteness Theorems.- Old or Heavy? Decaying Gracefully with Age/Weight Shapes.- Induction in Saturation-Based Proof Search.- Faster, Higher, Stronger: E 2.3.- Certified Equational Reasoning via Ordered Completion.- JGXYZ - An ATP System for Gap and Glut Logics.- GKC: a Reasoning System for Large Knowledge Bases.- Optimization Modulo the Theory of Floating-Point Numbers.- FAME(Q): An Automated Tool for Forgetting in Description Logics with Qualified Number Restrictions.