This paper studies the effects of trade policy uncertainty on the extensive and intensive margins of trade for a sample of 65 exporters at the Harmonized System six-digit level. The paper measures trade policy uncertainty as the gap between binding tariff commitments under trade agreements (multilateral and regional agreements) and applied tariffs -- what is also known as tariffs' water. The results show that trade policy uncertainty is an important barrier to exports and its effects are heterogeneous. On average and at the current level of tariff commitments, the paper estimates that the elimination of water, without any change of the applied tariff, would increase the probability of exporting by 6 percent and trade volumes by 1.3 percent. The negative impact of trade policy uncertainty on export participation is higher for countries with low-quality institutions and in the presence of global value chains. For a sample of new acceding countries, the analysis finds that removing water would boost the probability of trading by 50 percent and exports by 16 percent. The paper also estimates that the current system of commitments boosts trade by between 10 and 30 percent, compared with a world where at any moment tariffs could be raised to an arbitrarily high level.