Energy and Mass in Relativity Theory presents about 30 pedagogical papers published by the author over the last 20 years‎.‎ They deal with concepts central to relativity theory‎:‎ energy E‎,‎ rest energy E0‎,‎ momentum p‎,‎ mass m‎,‎ velocity v of particles of matter‎,‎ including massless photons for which v = c‎.‎ Other related subjects are also discussed‎.‎According to Einstein‎'‎s equation E0= mc2‎,‎ a massive particle at rest contains rest energy which is partly liberated in the nuclear reactions in the stars and the Sun‎,‎ as well as in nuclear reactors and bombs on the Earth‎.‎ The mass entering Einstein‎'‎s equation does not depend on velocity of a body‎.‎ This concept of mass is used in the physics of elementary particles and is gradually prevailing in the modern physics textbooks‎.‎This is the first book in which Einstein‎'‎s equation is explicitly compared with its popular though not correct counterpart E = mc2‎,‎ according to which mass increases with velocity‎.‎ The book will be of interest to researchers in theoretical‎,‎ atomic and nuclear physics‎,‎ to historians of science as well as t‎.‎‎.‎‎.‎