This compilation of selected papers offers valuable insights into the predominant challenges that occupied theoretical physicists over the most part of the last century. They will continue to be of interest to physicists and students pursuing advanced courses in physics and related fields.As known to many physicists, theoretical fancies are not always realized in experimental research. So, this book explores that important collaborative face of elementary particle physics through the author's work over the last six decades (1957-2017).The reader is invited to contemplate — If theory is to explain the observed world, and checks cannot be maintained, then what restraints should theorists face?