Although advances in genomics and transcriptomics techniques have led to significant improvements in cancer, proteomics techniques have recently become very important in cancer research. Proteomics has proven to be extremely useful and popular in cancer research, providing more comprehensive information on carcinogenesis and the identification of cancer-associated protein patterns. Although the effectiveness of clinical proteomics for patient management and clinical decision-making currently appears to be low, the search for cancer-related biomarkers using proteomics has a great potential for improving risk assessment, early detection, diagnosis, prognosis, treatment selection, and surveillance. Proteomics is a collection of technologies that focuses on all forms of proteins expressed in a cell, organ, or organism as a function of time, age, situation, and external factors. It plays an important role as a bridge between genomics and biology by providing information about what actually happens in the organism. As a result, there is an increasing interest in the use of proteomics techniques in cancer research. This section provides a detailed summary of proteomics technologies and applications used in current cancer research. However, this chapter also reviews an overview of lessons learned from currently validated protein biomarkers and previous proteomics research, what the limitations and challenges are in clinical proteomics applications, and how proteomics studies can be successfully transformed into clinical practice.