We posit that OECD buyers are in a continuous search for best quality suppliers from developing countries. We build a simple model of adverse selection and quality screening which captures this feature. The model predicts that diversification happens by "bouts", or temporary episodes, during which OECD buyers search for high-quality suppliers. Each diversification episode is followed by a phase of re-concentration on the best performers, until those fail (which happens stochastically), triggering new search phases. The model also shows that concentration across origin is highly volatile, especially for goods with high-quality heterogeneity. Finally, as the set of suppliers expands and buyers continue sampling, the overall trend is an increased diversification across time. We empirical explore these conjectures using OECD imports over time (1963-2006) and measuring their concentration across 250 origin countries at the product level (1,300 products). We provide strong empirical evidence corroborating the model predictions.