Apoptosis, a Greek descriptive term for falling leaves or petals, plays an important role in the progression of many diseases. Apoptosis is essential for the development and survival of multi-cellular organisms. Malignant diseases, including haematologic malignancies, are associated with defects in the cell death mechanism. These defects are not only important for the growth advantage of malignant clones, but when understood can be used for specific therapeutic targeting of malignant cells while sparing normal cells. The cellular and molecular mechanisms of apoptosis have been extensively demonstrated and are reviewed in this article. In this part of the review we focus on basic details of the apoptosis pathways, key players of the receptor-mediated apoptosis, and molecules involved in the cross-talk between individual apoptosis pathways and apoptosis regulation.