This thesis seeks to make sense of the change in the design of labor provisions in EU and US trade agreements over the years. What drives change? When are those changes weaker or stronger? In coining my "balance of promotion" argument, I argue that there is a causal relation between the strengthening of the commercial provisions of a preferential trade agreement (PTA) and the labor provisions therein. The rationale of my theory is that the greater the promotion of protection to exporters and multinationals via PTAs relative to the domestic protection of workers' rights in the negotiating partners, the greater the mobilization of domestic interests in favor of a stronger promotion of the rights of workers in those PTAs. I then hypothesize the degrees of change (low, moderate, high) according to the degree of mobilization of domestic interests in favor and against stronger labor provisions in PTAs. A process tracing of the EU and the US in the period between 2000 and 2013 confirms my hypotheses. To start exploring whether my theoretical argument is valid beyond the EU and the US, I also conduct plausibility probes of Canada and the Asia-Pacific region (Australia and Japan). My research offers an original contribution to the literature on the design of sustainable provisions in trade agreements.