This publication addresses some of the problems of maize improvement in the tropics and stresses that many can be solved through approaches involving the development and maintenance of broad-based gene pools and populations. The authors describe the system employed by CIMMYT's maize program, in which materials are assembled into gene pools, desirable materials advanced into populations of more specififc designation, and superior selections from these refined to produce high-yielding, stable varieties. The publication explains the role of international progeny testing in the refinement of maize germplasm and its distribution to