Cancer biomarkers can be used for a variety of purposes related to screening, prediction, stratification, detection, diagnosis, prognosis, treatment design, and monitoring of a therapeutic response. One of the most important characteristics of a given biomarker includes ease of collection allowing for a non-invasive approach and frequent sampling. Such samples may be obtained from serum or plasma, sputum, bronchoalveolar lavage, saliva, nipple discharge, pleural, or peritoneal effusions. Validation of different biomarkers is considered a mandatory method for useful evaluation. In this review, we highlight the clinical applicability of some cancer biomarkers, as well as future approaches for their development and collection, which may help guide clinicians and researchers. The role of liquid biopsies will also be summarized. Further studies using liquid biopsies are needed to elucidate the significance of various sources of biomarkers suitable for clinical application.