Poolish and levain are very similar in principle but a bit different in terms of composition. Levain refers to the part of a sourdough starter you add to a dough, while poolish is a yeasted pre-ferment that you add to a dough. Levain is typically made using wild yeast from the air while poolish is made with traditional baker's yeast. Both poolish and levain help the dough to rise and improve the flavor of the finished baked product.