Electron microscopy made possible the observation of the cellular/subcellular behaviour of tumour cells in metastasis. The marked progress in molecular biology has enabled the identification of the molecules involved in metastasis. Cellular diversity of metastatic tumour cells and host cells is extremely important not only in understanding the irregular metastatic behaviour of malignant tumour cells but also in elucidating why such populations may display different responses to therapy. There have been advances in research showing that clinical cancer metastasis will eventually be conquered, at least in part. This is evident from the fact that pathologists are now able to report the primary site of a metastatic tumour to clinicians. Hopefully, this book will enlighten people in the medical and research field regarding new research in cancer and the interested reader as well.