Small effects as a main problem in epidemiology.- Obstacles in evaluating small effects.- Towards good epidemiological practices.- Problems in detecting small effects in case-control and cohort studies.- Proposals and recommendations concerning small effects in case-control and cohort studies.- Comments on problems with small effects in case-control and cohort studies.- Small effects and the selection of study participants in case-control and cohort studies.- Commentary on small effects and the selection of study participants in case-control and cohort studies.- Confounding: Its role in weak associations.- Commentary on confounding: Examples of its influence in weak associations.- Bias in observational studies.- Research strategies for assessing epidemiolgic associations, in relation to the distribution and measurement of exposures.- Commentary on “Bias in observational studies”.- Small effects: Subgroup analysis and interaction.- Enhancing small risks in epidemiologic studies.- Is meta-analysis a valid approach to the evaluation of small effects?.- Commentary on meta-analysis.- Discussion remarks on meta-analysis.- Publication bias.- Concluding remarks.