This paper deals with the causes of the national differences in the level of scientific research. The level of scientific research is defined as the citation-impact of publications. The paper is based on a scrutinity of the explanation models presented in the relevant literature: 1) the number of talented researchers, 2) the amount of research funding, and 3) the conditions of research environment (modes of research funding, the time available for research, the infrastructure, the economies of scale, concentration, national and international cooperation, etc.). The explanatory power of the models is critically examined based on the results of empirical studies and mutual comparison. The paper is part of a research project “Why is Finland not successful in international science comparisons?”. The empirical part of the project is carried out by comparing Finland to Switzerland, the Netherlands, Denmark, Sweden and Norway. The research includes some ideas on how to improve Finland’s position in international science comparisons.