This paper draws on Marxist scholarship concerning the law's emancipatory potential to shed new light on the history of UK trade unions, and their relationship with law. It traces the historical development of UK trade union law from the nineteenth century to the present day with a view to illustrating the importance of considering not just the content, but also the form, of law, in explaining the role of law in shaping the development of the trade union movement, and in understanding the limits of law, including human rights law, when it comes to realising the emancipatory potential of trade unions in society today. It concludes with some observations about how legal and social actors might make use of their understanding of the legal form when it comes to harnessing the law as part of their political strategies.