Although it is not uncommon to find expressions of doubt about the need to use a formal taxonomy to classify trace fossils, ichnotaxonomic classification is an unavoidable companion to preservational and ethological schemes. If a formal name is available, simple descriptors (e.g. vertical burrows and meniscate traces) should be avoided. The ichnotaxonomic classification, albeit imperfect, provides the best common ground on which to base more theoretical elaborations and practical applications (Buatois et al., 2002a). In any case, in modern ichnology contrasting philosophical perspectives have been adopted to classify trace fossils. However, exchange of ideas during and after the 1998, 2002, 2006, and 2010. Workshops on Ichnotaxonomy have resulted in a growing consensus among practicing ichnologists (Bertling et al., 2006). In this chapter, we turn our attention into the theoretical and practical aspects involved in classifying trace fossils from a taxonomic standpoint. We first address some philosophical problems involved in this approach. Then, we focus on a detailed review of the different ichnotaxobases currently in use and the problems associated with compound and composite trace fossils. Subsequent to that, we move on to some recent ideas and proposals with respect to the uses of hierarchies in trace-fossil taxonomy and the peculiarities of vertebrate ichnotaxonomy. Finally, we review some practical aspects involved in the recognition of trace fossils in both outcrops and cores.