The fundamental characteristic of the MK classification system is the definition of an array of standard stars, arranged in two dimensions. New techniques and an enormous increase in observational and theoretical work since the original catalog in 191+3 have in various ways complicated the problem of spectral classification. Problems discussed are: 1.) The re-examination of standard stars; 2.) The optimum dispersion and the limits of dispersion for MK classification; and 3.) Some groups of peculiar objects.