This study compares the journal coverage of Web of Science (WoS) and Scopus, focusing on fields, countries, and languages. The research uses Ulrich's extensive periodical directory as a reference. Results show that both databases have biases favoring Natural Sciences and Engineering, Biomedical Research, and English-language journals, while underrepresenting Social Sciences and Arts and Humanities. Despite these biases, their coverage differs significantly. WoS and Scopus both have similar biases, but Scopus covers more journals than WoS. The coverage overlap between the two databases is also significant, with Scopus covering more exclusive journals than WoS. The study also finds that the coverage of journals by country is skewed, with countries like the United States, the United Kingdom, the Netherlands, France, Germany, and Switzerland being overrepresented in both databases. The coverage by language is also skewed, with English being overrepresented in both databases, while other languages are underrepresented. The study concludes that using WoS and Scopus for research evaluation introduces biases that favor Natural Sciences and Engineering as well as Biomedical Research to the detriment of Social Sciences and Arts and Humanities. Similarly, English-language journals are favored to the detriment of other languages. These biases should be taken into account when assessing scientific activities. The study also suggests that there may be better tools for research evaluation, such as Google Scholar, which provides free access to scholarly documents of all types, language, and for all fields. However, Google Scholar has been criticized for its sporadic coverage of non-English literature and lack of transparency. The study also highlights the importance of using field-specific or national citation indexes for more accurate research evaluation.This study compares the journal coverage of Web of Science (WoS) and Scopus, focusing on fields, countries, and languages. The research uses Ulrich's extensive periodical directory as a reference. Results show that both databases have biases favoring Natural Sciences and Engineering, Biomedical Research, and English-language journals, while underrepresenting Social Sciences and Arts and Humanities. Despite these biases, their coverage differs significantly. WoS and Scopus both have similar biases, but Scopus covers more journals than WoS. The coverage overlap between the two databases is also significant, with Scopus covering more exclusive journals than WoS. The study also finds that the coverage of journals by country is skewed, with countries like the United States, the United Kingdom, the Netherlands, France, Germany, and Switzerland being overrepresented in both databases. The coverage by language is also skewed, with English being overrepresented in both databases, while other languages are underrepresented. The study concludes that using WoS and Scopus for research evaluation introduces biases that favor Natural Sciences and Engineering as well as Biomedical Research to the detriment of Social Sciences and Arts and Humanities. Similarly, English-language journals are favored to the detriment of other languages. These biases should be taken into account when assessing scientific activities. The study also suggests that there may be better tools for research evaluation, such as Google Scholar, which provides free access to scholarly documents of all types, language, and for all fields. However, Google Scholar has been criticized for its sporadic coverage of non-English literature and lack of transparency. The study also highlights the importance of using field-specific or national citation indexes for more accurate research evaluation.