This survey is meant to give an introduction to elementary techniques used for designing I/O-efficient algorithms. We do not intend to give a complete survey of all state-of-the-art techniques; but rather we aim to provide the reader with a good understanding of the most elementary techniques. Our focus is on general techniques and on techniques used in the design of I/O-efficient graph algorithms. We include the latter because many abstract data structuring problems can be translated into classical graph problems. While this fact is of mostly philosophical interest in general, it gains importance in I/O-efficient algorithms because random access is penalized in external memory algorithms and standard techniques to extract information from graphs can help when trying to extract information from pointer-based data structures.