This report describes a method for analysing failure and maintenance data for a population of complex repairable systems with the aim of improving maintenance efficiency. It is part of a Ph.D. study, titled "Maintenance and replacement strategies for complex systems", the objective of which is to analyze failure and maintenance data using mathematical and statistical models in order to improve maintenance procedures in the Danish Defence. The first part of the report introduces the maintenance planning problem and presents an overview of models for reliability, failure processes, and maintenance planning. This overview is structured to highlight the process of choosing a proper model for a given data set, focusing on different measures of time and the data requirements for the different models. The second part of the report describes the analysis of two data sets from the Danish Defence. The data sets are analyzed using a graphical method, Nelson-Aalen plots, as well as multiplicative intensities models with proportional intensities regression, which is a parametric model.