This book constitutes the refereed proceedings of the 6th International Workshop on Algorithms and Data Structures, WADS'99, held in Vancouver, Canada, in August 1999.The 32 revised papers presented were carefully reviewed and selected from a total of 71 submissions. Among the topics covered are data structures and algorithmic aspects in a variety of areas such as graph theory, networking, load balancing, optimization, approximation, sorting and searching, pattern matching, information retrieval, computational mathematics, etc.