This volume contains 21 research articles on topics in universal algebra and lattice theory. It continues the series "Contributions to General Algebra", Volumes 1-10, published from 1979 to 1998. All submitted manuscripts have been refereed by an international board. The papers were presented at the 56th Conference on General Algebra, June 12-14, 1998, at the University of Olomouc, and at the Summer School '98 on Universal Algebra and Ordered Sets, August 31 - September 5, 1998, at Velke Karlovice, Czech Republic.