An extensively curated database of high-resolution tandem mass spectra that are arranged into spectral trees. mzCloud represents a key conceptual shift towards a modern mass spectral database. This tool is a scientifically solid and computationally robust platform for the identification of small molecules using tandem mass spectrometry. It offers a web-based interface providing a number of search, visualization and data processing tools. It also comprises curated databases of high and low resolution MSn spectra acquired under a number of experimental conditions which address the spectra reproducibility problem.