Companies organize into groups for various reasons. From a legal point of view a group of companies is an interesting subject of research, since it is a topic that affects various different areas of law. Also, groups of companies are often transnational so that the parent company and the subsidiaries operate in different jurisdictions. Legal provisions concerning groups are, however, quite national and therefore it is important to know, how groups of companies are regulated in different countries. In light of this background, the purpose of this article is to give a comprehensive overview of the law regarding Finnish groups of companies. The research has been conducted so that the initial focus is on company law and cooperative law-related issues, as well as accounting. After the main analysis, there is a brief overview of other group-specific rules. Here the focus is on tax law, insolvency law and labor law matters. Finally, the research findings are summarized in a brief conclusions section. The article is part of the International Academy of Comparative Law’s “Groups of Companies” research project and the research findings will be presented at the Academy’s XXth International Congress 2018, Fukuoka, Japan.