Three important properties associated with a classification of any group of organisms are diagnosability, monophyly and resolution. In this chapter we explore the interrelationships between these three properties in the context of cryptic taxa, here defined as a clade with no obvious diagnostic morphological support. We present the view that the number of nodes on a phylogenetic tree of all flowering plants that have morphological diagnostic support is less than five percent; as such, cryptic nodes are much more common than non-cryptic nodes. Because of this, we suggest that the phrase ‘cryptic nodes’ is a preferable description as opposed to cryptic taxa because taxa in the sense of traditional classifications are generally diagnostic. By reference to a global taxonomic study of the genus Ipomoea, we discuss the role of diagnosability at various scales including major infrageneric clade, genus and species. We demonstrate that the level of diagnosability for Ipomoea is relatively low, therefore making cryptic nodes the rule and not the exception. We provide several examples of such cryptic nodes, detail how we discovered them and place them in a wider conceptual framework of diagnosability in angiosperms.