To Professor Haïm Brezis, with profound admiration, on the occasion of his 70th birthday. Avec mes sincères remerciements, pour votre amitié, vos enseignements, et votre exempleédifiant. Votreétudiant, Michel.Key words and phrases. Fractal set, fractal drum, relative fractal drum, fractal zeta functions, distance zeta function, tube zeta function, geometric zeta function of a fractal string, Minkowski content, Minkowski measurability, upper box (or Minkowski) dimension, complex dimensions of a fractal set, relative fractal drum, holomorphic and meromorphic functions, abscissa of convergence, quasiperiodic function, quasiperiodic set, order of quasiperiodicity, spectral asymptotics of fractal drums.* The work of Michel L. Lapidus was partially supported by the US National Science Foundation (NSF) under the research grants DMS-0707524 and DMS-1107750, as well as by the Institut des Hautes Etudes Scientifiques (IHES) in Paris/Bures-sur-Yvette, France, where the first author was a visiting professor in the Spring of 2012 while part of this work was completed.† Goran Radunović and DarkoŽubrinić express their gratitude to the Ministry of Science of the Republic of Croatia for its support, as well as to the University of Zagreb for supporting a visit of the second author to the University of California, Riverside, in the Spring of 2014 during which a part of this article was completed. Abstract. The theory of 'zeta functions of fractal strings' has been initiated by the first author in the early 1990s, and developed jointly with his collaborators during almost two decades of intensive research in numerous articles and several monographs. In 2009, the same author introduced a new class of zeta functions, called 'distance zeta functions', which since then, has enabled us to extend the existing theory of zeta functions of fractal strings and sprays to arbitrary bounded (fractal) sets in Euclidean spaces of any dimension. A natural and closely related tool for the study of distance zeta functions is the class of 'tube zeta functions', defined using the tube function of a fractal set. These three classes of zeta functions, under the name of 'fractal zeta functions', exhibit deep connections with Minkowski contents and upper box dimensions, as well as, more generally, with the complex dimensions of fractal sets. Further extensions include zeta functions of relative fractal drums, the box dimension of which can assume negative values, including minus infinity. We also survey some results concerning the existence of the meromorphic extensions of the spectral zeta functions of fractal drums, based in an essential way on earlier results of the first author on the spectral (or eigenvalue) asymptotics of fractal drums. It follows from these results that the associated spectral zeta function has a (nontrivial) meromorphic extension, and we use some of our results about fractal zeta functions to show the new fact according to which the upper bound obtained for the corresponding abscissa of meromorphic convergence is optimal. Finally...