PrefaceThis book intends to deepen the study of the fractional calculus, giving special emphasis to variable-order operators.Fractional calculus is a recent field of mathematical analysis and it is a generalization of integer differential calculus, involving derivatives and integrals of real or complex order (Kilbas, Srivastava and Trujillo, 2006;Podlubny, 1999). The first note about this ideia of differentiation, for non-integer numbers, dates back to 1695, with a famous correspondence between Leibniz and L'Hôpital. In a letter, L'Hôpital asked Leibniz about the possibility of the order n in the notation d n y/dx n , for the nth derivative of the function y, to be a non-integer, n = 1/2. Since then, several mathematicians investigated this approach, like Lacroix, Fourier, Liouville, Riemann, Letnikov, Grünwald, Caputo, and contributed to the grown development of this field. Currently, this is one of the most intensively developing areas of mathematical analysis as a result of its numerous applications. The first book devoted to the fractional calculus was published by Oldham and Spanier in 1974, where the authors systematized the main ideas, methods and applications about this field (Mainardi, 2010).In the recent years, fractional calculus has attracted the attention of many mathematicians, but also some researchers in other areas like physics, chemistry and engineering. As it is well known, several physical phenomena are often better described by fractional derivatives (Herrmann, 2013;Odzijewicz, Malinowska and Torres, 2012a; Sheng, 2012). This is mainly due to the fact that fractional operators take into consideration the evolution of the system, by taking the global correlation, and not only local characteristics. Moreover, integer-order calculus sometimes contradict the experimental results and therefore derivatives of fractional order may be more suitable (Hilfer, 2000).In 1993, Samko and Ross devoted themselves to investigate operators when the order α is not a constant during the process, but variable on time: α(t) ). An interesting recent generalization of the theory of fractional calculus is developed to allow the fractional order of the derivative to be non-constant, depending on time (Chen, Liu and Burrage, VI 2014; Odzijewicz, Torres, 2012b, 2013a). With this approach of variable-order fractional calculus, the non-local properties are more evident and numerous applications have been found in physics, mechanics, control and signal processing (Coimbra, Soon and Kobayashi, 2005; Ingman and Suzdalnitsky, 2004;Odzijewicz, Malinowska and Torres, 2013b; Ostalczyk et al., 2015;Ramirez and Coimbra, 2011; Rapaić and Pisano, 2014; Valério and Costa, 2013).Although there are many definitions of fractional derivative, the most commonly used are the Riemann-Liouville, the Caputo, and the Grünwald-Letnikov derivatives. For more about the development of fractional calculus, we suggest (Samko, Kilbas and Marichev, One difficult issue that usually arises when dealing with such fractional operators, is the extreme di...