  There is a certain family of conformally invariant first order elliptic
operators on Riemannian spin manifold which include Dirac operator as its first
and simplest member. Their general definition is given and their basic
properties are described. A special attention is paid to the Rarita-Schwinger
operator the second simplest operator in the row. Its basic properties are
described in more details. In the last part indices of discussed operators are
computed.
