This book constitutes the refereed proceedings of the 6th International Conference,on Theory and Practice of Natural Computing, TPNC 2017, held in Prague, Czech Republic, December 2017. The 22 full papers presented in this book, together with one invited talk, werecarefully reviewed and selected from 39 submissions. The papers are organized around the following topical sections: applications of natural computing; evolutionary computation; fuzzy logic; Molecular computation; neural networks; quantum computing.