Biostimulant is loosely defined as an organic material and/or microorganism, which is applied to enhance nutrient uptake, stimulate growth, and enhance stress tolerance or crop quality. Extracts from algae or from different plant/microbial material have been reported to confer beneficial effects on growth and stress adaptation. Many primary and secondary metabolites have been reported as possible candidates for improving plant growth, including plant hormones, phenolic compounds and amino acids among others. In our research group we are focus on the identification of these metabolites in biostimulants from different extracts, and in parallel the quantitative changes of the same metabolites in the different crops subjected to diverse growth conditions. This approach allows us to understand the role that the identified compounds are playing as plant growth regulate and/or stress alleviators. The wide experience in the study of plant growth regulators and the use of leading technologies based on ultra-high performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS) have permitted us to increase the number of metabolites detected as well as simplification of purification protocol and the miniaturization of the sample preparation methods used.