The aim of this book is to introduce the reader to the person-oriented approach. It is hoped that the book will be complete in the sensethat we present both the basic theory behind the approach, selected methods for person-oriented analysis (cross-sectional and longitudinal), a statistical package, SLEIPNER, for carrying out such analyses (including the program available on the Web), and empirical examples. Although we believe the book contains enough information to enable the careful reader to apply a person-oriented approach in her or his research, the book is by no means exhaustive. We sometimes only indicate methodological approaches other than those treated here and guide the reader to the appropriate literature.