This book is about Einstein's Summation Notation (ESN). It explains the rules for correct use of ESN-notation, covering the Kronecker-symbol and the Levi-Civita-symbol. Examples are taken from Linear Algebra (vector - and matrix - calculation) as well as functional analysis (diferential operators gradient, curl, divergence). The book aims at students of physics who want to deal with the Special - and General Theory of Relativity by Albert Einstein, as the correct use of ESN in this area is essential.