Streptomyces has great potential to produce novel natural products, but most of their biosynthetic gene clusters (BGCs) are silent or expressed in an extremely low level under laboratory conditions. Signal molecules are used to regulate phenotypic differentiation and biosynthesis of secondary metabolites. Manipulation of genes encoding the synthetases or receptors for signal molecules, or addition of exogenous signal molecules into the fermentation medium relieves the repression of signal molecule receptors on the expression of Streptomyces BGCs. This strategy can be used to activate silent BGCs to discover novel natural products or to increase the titer of known secondary metabolites. In this review we used γ-butyrolactones (GBLs) and γ-butenolides as examples to summarize the application of signal molecules in the discovery and titer improvement of Streptomyces secondary metabolites, to provide a reference for the development of microbial natural products.