Quantum Mechanics in Matrix Form Preface and Introduction Quantum Theory Before 1925 Heisenberg 1925 Expansion of the Matrices Method Observables and Uncertainty Relations Harmonic Oscillator Pauli and the Hydrogen Atom Spin Atoms in Electromagnetic Fields Systems of Several Particles Equivalence of Matrix with Wave Mechanics Relativistic Quantum Mechanics. This book gives an introduction to quantum mechanics with the matrix method. Heisenbergs matrix mechanics is described in detail. The fundamental equations are derived by algebraic methods using matrix calculus. Only a brief description of Schrödingers wave mechanics is given (in most books exclusively treated) to show their equivalence to Heisenbergs matrix method. In the first part the historical development of Quantum theory by Planck Bohr and Sommerfeld is sketched followed by the ideas and methods of Heisenberg Born and Jordan. Then Paulis spin and exclusion principles are treated. Paulis exclusion principle leads to the structure of atoms. Finally Dirac´s relativistic quantum mechanics is shortly presented. Matrices and matrix equations are today easy to handle when implementing numerical algorithms using standard software as MAPLE and Mathematica.