Cancer is a collection of diseases that can affect basically every organ of our body, all of which have in common uncontrolled cellular growth. The cells forming our body have the potential to grow in the context of wound healing or for the constant replacement of cells in our blood, skin or intestine. Behind every newly diagnosed malignant tumor in adulthood there is an individual history of probably 20 or more years of tumorigenesis. Therefore, malignant tumor formation often takes time making cancer in most cases to an aging-related disease that we seem not to be able to evade. However, tumorigenesis is dependent on multiple environmental influences, many of which we have under control by lifestyle decisions, such as retaining from smoking, selecting healthy food and being physically active. Thus, cancer preventive interventions are the most effective way to fight against cancer.  This textbook wants not only to describe basic mechanisms leading to cancer but also to provide the readers with a more holistic view including cancer surveaillance mechanisms of the immune system. We will place these insights in the context of the personal consequences of everyone's lifestyle decisions. The content of the book is linked to the lecture course in "Cancer Biology", which is given by Prof. Carlberg since 2005 at the University of Eastern Finland in Kuopio. Moreover, biological processes explained in this book will be set into a clinical context using the experience of Dr. Velleuer in the daily care in oncology. This book also relates to the textbooks "Mechanisms of Gene Regulation: How Science Works" (ISBN 978-3-030-52321-3), "Human Epigenetics: How Science Works" (ISBN 978-3-030-22907-8) and "Nutrigenomics: How Science Works" (ISBN 978-3-030-36948-4), the studying of which may be interesting to readers who like to get more detailed information.