This book discusses the topic of clustering on data mining. Some clustering methods are mentioned in this section of the book. The emphasis of the clustering topic is on K-Means Clustering method. In this book is also included a K-Means clustering case study applied to patient data at a hospital. The case studies mentioned in this book are the results of research done by the author funded by the Ministry of Research, Technology and Higher Education Republic of Indonesia. Clustering impelementation is done by using WEKA software.