Recognition of hand-drawn, diagrammatic sketches is a very active research field, since it finds a natural application in a wide range of domains, such as engineering, software design, and architecture. However, it is a particularly difficult task since the symbols of a sketched diagram can be drawn by using a different stroke-order, -number, and -direction. The difficulties in the recognition process are often made harder by the lack of precision and by the presence of ambiguities in messy hand-drawn sketches. In this article we present a brief survey on sketch understanding techniques and tools. We first present major problems that should be considered in the construction of on-line sketch recognizers. We analyze representative works for the recognition of freehand shape and describe several shape description languages for the automatic construction of sketch recognizers.