The Kiel classification of non-Hodgkin lymphomas (NHL) identifies distinct entities by morphological and immunological criteria, subdivided into those of low and high grade malignancy and of B- or T-cell subtype. Numerous clinical studies have proven the clinical and prognostic relevance of this classification scheme although the degree of malignancy alone cannot be automatically equated with prognosis. Thus, histological low grade malignancy correlates with favorable response to therapy in all, but with long term survival only in some clearly defined low grade NHL and final cure is achieved in none of them so far. High grade malignancy corresponds to the aggressive spontaneous course of these NHL but is contrasted with the mostly favorable response to therapy and the chance of cure. The immunological T-cell subtype is clearly associated with an inferior prognosis. Additional biological features of NHL, namely chromosomal aberrations and the status of clonality will contribute to a more differentiated view of NHL and will possibly prove to bear prognostic potential.