This book serves as the primary text of the regular course on algorithms, data structures and complexity offered to 2nd year students of computer science at the Technical University, Budapest (about 250 students yearly). It covers the rudiments of sorting, data structures for searching (balanced search trees, hash tables), data compression, graph algorithms (shortest paths, DFS, BFS, mini-mum cost spanning trees, matching, maxi-mum flows in networks). Subjects related to complexity include Turing machines, recursivity, undecidable languages, P, NP, RP. The book discusses some general algorithm-desgin techniques through examples (branch and bound, approximation algorithms, dynamic programming, randomisation, preconditioning). The material has been tested in classrooms and received positive feedback from students and professionals alike.