Since the introduction of the concept of aplastic anemia (AA) by Paul Ehrlich in 1888 and despite the current better understanding of the underlying mechanisms involved in this disease, a clear delimitation among BM failure syndromes is still a matter of debate. The diagnosis of AA can be difficult basically due to the overlapping morphological characteristics with other BM failure disorders. This paper reviews critical data relevant to the diagnosis of acquired AA and recommends work out steps and main considerations to determine severity and characterization of the disease. The diagnostic challenge in the differentiation between AA and hypoplastic myelodysplastic syndromes is also addressed. The definition of the response criteria to treatment belongs to the diagnostic tasks and it is included in this review as well as an overview of novel tools for the diagnosis of AA.