The need to understand the impact of the environment on the fluctuations in fisheries yield in the Northeast Atlantic was a major motivation for the creation of ICES. From the very beginning, two main research tasks were talen on: l) the establishment of relationships between the distribution and behaviour of fish and environmental characteristics for the purpose of real-time and short-term predictions and advice about where and when the fishing fleets could increase their catches, and 2) the influence of environmental factors on fish stock parameters/variables such as recruitment, growth, maturation, and mortality in order to advise on optimum yields and forecast yield fluctuations. While the purpose of the first of these tasks was rendered unnecessary some decades ago by developments in fish finding and communication systems in the commercial fleets, the second task remains a major topic for study. Until a few decades ago, knowledge of the relationship between fish and environmental conditions was largely qualitative and based almost exclusively on field studies which provided information on "associated phenomena", in some instances quantified by correlations where the underlying mechanisms were hypothesized. During the 1960s and 1970s, efforts to describe "cause and effect" and to quantify the relationships increased considerably. These efforts included studies of species and stock interactions, ecosystem modelling, and field and laboratory experiments with instrumentation enabling observations of fish and their offspring as well as environmental variables over wide ranges of size, space, and time scales. The results of such process-related investigations, coupled with observations from long time series of environmental data and life history tables for fish stocks form the basis for our present understanding of how environmental conditions influence stocks and yields. For some stocks, such knowledge has been included in the regular stock assessments carried out by ICES in recent years.