Effective maintenance has become increasingly important the last few decades. Competition is increasing because of globalisation. Therefore, production is confronted with increasing requirements. In particular, machinery and plants have to produce faster and in greater volume. Nowadays, the high availability of equipment is a prerequisite to compete. In recent decades, maintenance has become an own business area. The Lean Management method Total Productive Management (TPM) provides a guideline for effective maintenance, but the maintenance process itself is not adequately described in the literature. However, it is an efficient means of addressing unplanned maintenance tasks. This is the reason for defining a universal maintenance process which is valid for any kind of maintenance. The creation of the reference model for the maintenance process is presented in this book.