This review covers work on fiber optic chemical sensors and biosensors (FOCS) published in the time between October 2015 and October 2019 and is written in continuation of previous reviews. In exceptional cases, older references were cited. Data were electronically searched in SciFinder, MedLine, and Google Scholar. Additionally, the authors collected references from analytical and sensor journals over the past 5 years. The focus is on chemical sensors and biosensors for defined chemical, environmental, or biochemical species on detection schemes and on new materials for analyte recognition and for signal transduction. We have not included certain kinds of work for various reasons. The following work was not included: (a) Papers related to optical engineering or on the optics of waveguides only; (b) work on FOCS that obviously have been rediscovered; (c) FOCS for monitoring purely technical processes such as injection molding, extrusion, or oil drilling (even though these may represent important applications of optical fiber technology); (d) sensors for purely physiological parameters such as respiration, hematocrit, or heart beat rate; (e) sensors for purely physical parameters such as temperature, turbidity, or pressure. While many articles deserve being treated in more than one section because they excel in more than one field, they had to be allocated to a single section for reasons of shortness.