Longitudinal cancer biomarker studies are frequently regarded as difficult, probably because there is no consensus on how to design and conduct these types of investigations.To aid in the appropriate development, the European Group on Tumour Markers proposes a set of guidelines to design, conduct, and report longitudinal monitoring trials.The proposals refer to biochemical compounds denoted as cancer biomarkers or tumor markers measured in serum,plasma or blood obtained from patients with solid tumors.The MONITOR proposals are designed analogous to systems already in use for new drug development and inspired by the following guidelines: RECIST (version 1.1) for response evaluation criteria in solid tumours, CONSORT for randomized controlled trials, REMARK for prognostic markers,STARD for diagnostic markers, MOOSE for meta-analysis of observational studies in medicine, QUORUM for metaanalysis of randomized trials, and STROBE for cohort and case control studies.The EGTM proposes, i) trial phases Ⅰ-Ⅳ for cancer biomarker monitoring studies, ii) guiding principles to generate assessment criteria for serial marker measurements iii) guiding principles for cancer biomarker response evaluation similar to the evaluation of disease status in drug trials, and iv) a scheme to classify the cancer biomarker response status into true and false results.A framework is proposed for conducting longitudinal trials that may help to clarify the evidence for utilizing cancer biomarkers as monitors.Longitudinal cancer biomarker monitoring trials have many facets to consider and several pitfalls to avoid.Accordingly, these types of investigations should be based on multidisciplinary collaboration.Codevelopment of oncology drugs and biomarker use may benefit from longitudinal marker studies that parallel the evaluation of the response to cancer therapy.