This chapter is concerned with the efficient implementation of “low-level” morphological transformations [25,38]. The qualifier “low-level” used here means that we deal with the implementation of transformations which serve as elementary bricks when solving practical image analysis problems. This does not mean that these transformations are simple, or cannot be decomposed into simpler ones; on the contrary, some of the operations considered in this chapter (e.g., skeletons, watersheds, propagation functions) are complex, both to define and to compute! However, from a user’s perspective, these transformations share the characteristics of being easily and intuitively understandable; for example, watersheds extract from a gray-level image the crest lines that are located between the minima, top-hat transformations extract thin and light (or dark) regions, and skeletons reduce binary shapes to their medial axes.