Proteomics is the analysis of proteins of a part of or a complete cell, tissues, or organism under given conditions. Mass-spectrometry based approaches are used to analyze proteins either in their intact form (top-down proteomics) or as digested peptides (bottom-up proteomics). Even though in principle a top-down approach can fully sequence the protein of interest, this method is not widely used due to several limitations; therefore, bottom-up approaches are more popular in a wide range of applications encompassing both simple and complex samples. The recent advances in mass spectrometry-based proteomics methods have increased the capacity for identification of proteins, as well as aided in the unraveling of cellular signaling networks and improved diagnosis/molecular understanding of disease mechanisms. In answering biological and clinical questions, a list of proteins is usually insufficient; a quantitative state of a proteome is needed for significant insight on disease states or other experimental conditions. Complex digested peptides result in a pool of peptides with varying chemical and physical properties, differing in charge state, hydrophobicity, amino acid sequence, and PTMs. Thus, since the peptide quantitation can not be measured directly from signal intensities this has led to development of various quantitative techniques. Current methods for protein quantitation fall under two main categories: (1) isotope labeling methods (chemical, metabolic and isobaric) which use heavy components; and (2) label-free methods, which use peptide or peptide fragments to obtain quantitative data. The goal of this collection is to feature quantitative mass spectrometric methods in biological samples.