Purpose: The aim of this brief review was to present an overview of noninvasive markers in trained to professional endurance athletes that can reflect a state of functional overreaching. Methods: A systematic literature search was conducted in the PubMed, Scopus, and PsycINFO databases. After screening 380 articles, 12 research papers were included for the systematic review. Results: Good consensus was found between the different papers in which noninvasive parameters were able to reflect a state of functional overreaching. Changes in power output (PO), heart rate (HR; [sub]maximal and HR recovery), rating of perceived exertion, and scores in the Daily Analysis of Life Demands for Athletes (DALDA) and/or Profile of Mood States (POMS) were shown to be able to reflect functional overreaching, whereas changes in maximal oxygen uptake and HR-variability parameters were not. Conclusion: Functional overreaching within a maximal performance test was characterized by a decrease in peak PO and a lower maximum HR, whereas a lower mean PO and a lower HR were observed during time trials. Changes in parameters during a standardized submaximal test when functionally overreached were characterized by a higher PO at a fixed HR or a lower HR at a fixed intensity, higher rating of perceived exertion, and a faster HR recovery. Although both the DALDA and POMS were able to reflect functional overreaching, the POMS was not able to differentiate this response from acute fatigue, which makes it unsuitable for accurately monitoring functional overreaching.