Discovery of candidate biomarkers within the entire proteome is one of the most important and challenging goals in proteomic research. Mass spectrometry-based proteomic is a modern and promising technology for semiquantitative and qualitative assessment of proteins, enabling protein sequencing and identification with exquisite accuracy and sensitivity. For mass spectrometry analysis, protein extractions from tissues of interest or body fluids with subsequent protein fractionation represent an important and unavoidable step in the workflow for biomarker discovery. The aim of our chapter is to provide practical lab procedures for sample digestion and protein fractionation for subsequent mass spectrometry analysis.