Despite the passage of half a century since 'demographic transition theory' first appeared in English-language demographic literature, many would agree that relatively little progress has been made toward refinement and testing of leading theoretical ideas, or towards theoretical consensus. There has rather been a proliferation of untested theories, partially overlapping and often associated with one individual, discipline, or school of thought. It is argued here that lack of clarity, precision and rigor in the statement, criticism, and use of theories of fertility decline is a major cause of the lack of theoretical progress. The greater use of tools for the computer modeling of complex dynamic systems is seen as a partial remedy. Emphasis is on the use of computer modeling (simulation) as a theoretical tool.The current status of demographic theories of fertility decline leaves something to be desired. "Classic" transition theory has been "reconsidered," "restated" (more than once), and recently rejected as an intellectual enterprise allegedly driven by political rather than scientific concerns and informed by outdated views of social science methodology (Szreter, 1993). Micro-economic ("demand") theories of fertility have fallen victim to "iconoclasm" at the hands of Cleland and Wilson (1987), who find ideational "rather than" economic/structural factors to be "the fundamental force behind fertility decline" (p.1). Indeed, theoretical writing has tended toward polarization between those who attribute fertility decline largely to economic factors and those who attribute it largely to cultural factors (Johansson 1993), a polarization epitomized by the title (if not the substance) of Hammel's recent paper (1993) -