This special issue of Algorithmica contains the journal version of selected papers from the ninth Latin American Theoretical Informatics Symposium (LATIN), which was held in the city of Oaxaca, Mexico in April 2010. LATIN was launched in 1992 to foster the interaction between the Latin-American theoretical computer science community and computer scientists around the world. The conference, which is international in scope, is now held every other year, and LATIN’10 was the ninth of a series, after Sao Paulo, Brazil (1992); Valparaiso, Chile (1995); Campinas, Brazil (1998); Punta del Este, Uruguay (2000); Cancun, Mexico (2002); Buenos Aires, Argentina (2004); Valdivia, Chile (2006); and Buzios, Brazil (2008). The conference proceedings were published by Springer in the Lecture Notes in Computer Science series, as they have been since inception. There were fifty-six papers presented in LATIN’10, and Volume 6034 of LNCS is devoted to them. A set of papers were selected from those presented in LATIN’10 and invited to submit an extended version for this issue. Each of these papers went through a careful selection and refereeing process. They provide a wide view of the different subfields of theoretical computer science within the scope of LATIN. We are grateful to the authors for their valuable contributions, as well as the reviewers for their effort and dedication. The papers in this issue are: • Pairs of Complementary Unary Languages with “Balanced” Nondeterministic Automata. Viliam Geffert, Giovanni Pighizzini. • The Complexity of Counting Eulerian Tours in 4-Regular Graphs. Qi Ge, Daniel Stefankovic. • Minimum-perimeter Intersecting Polygons. Adrian Dumitrescu, Minghui Jiang.