This Special Issue of RAIRO, Theoretical Informatics and Applicationsis devoted to full versions of selected papers from the workshop WordAvoidability, Complexity and Morphisms which took place in Turku(Finland) on July 2004, as a satellite event of the conferenceICALP'2004.
The topics of this one day workshop concern particular aspects ofCombinatorics on Words: string pattern avoidability, complexities offinite or infinite words, and free monoids morphims.
The scientificprogram of the worshop consisted of two invited lectures given byA.E. Frid and J. Shallit as well as presentations of 8 contributedpapers selected by the Program Committee from a total of 12submissions.
The Program Committee consisted of :A. Carpi (Perugia),J. Cassaigne (Marseille),J. Currie (Winnipeg), I. Petre (Turku),G. Richomme (Amiens),A. M. Shur (Ekaterinburg), andP. Séébold (Montpellier). 
This special issue contains the full text of five of the paperspresented at the workshop. All of them were normally refereed andrevised.
I would like to thank everybody who contributed to the workshop andthis issue including invited speakers, members of the programCommittee, ICALP'2004's organization, authors and all referees of paperssubmitted to the worshop and this special issue. I am indebtful toJ. Kärhumaki and J. Berstel for thinking of me to organize thisworkshop. Special thanks to P. Séébold for all his advices. Finally Iam grateful to C. Choffrut, the editor-chief of the RAIRO-TIA foraccepting to publish this special issue.