Operator Theory A one sentence definition of operator theory could be: The study of (linear) continuous operations between topological vector spaces these being in general (but not exclusively) Fréchet Banach or Hilbert spaces (or their duals). Operator theory is thus a very wide field with numerous facets both applied and theoretical. There are deep connections with complex analysis functional analysis mathematical physics and electrical engineering to name a few. Fascinating new applications and directions regularly appear such as operator spaces free probability and applications to Clifford analysis. In our choice of the sections we tried to reflect this diversity. This is a dynamic ongoing project and more sections are planned to complete the picture. We hope you enjoy the reading and profit from this endeavor General aspects of quaternionic and Clifford analysis Further developments of quaternionic and Clifford analysis Infinite dimensional analysis Non commutative theory Multivariable operator theory Reproducing kernel Hilbert spaces de Branges spaces Indefinite inner product spaces Schur analysis Linear system theory