This book has been prompted by recent advances in the safe prevention of thromboembolism by subcutaneous heparin prophy­ laxis, in particular postoperativt>ly. It has been correctly called by S. Sherry a major breakthrough in medicine. Although thrombo­ embolism was first recognized by Laennec in 1819 and defined by Virchow in 1846, its development was not well understood and its prevention escaped our best efforts until now. This all-too-com­ mon, sudden, unexpected and unwanted form of morbidity and mortality, always referred to in the surgical patient as postoper­ ative pulmonary thromboembolism, has now become the major complication of all surgery. However, it occurs with equal fre­ quency in hospitalized non-operative patients as well, so the latter are also in need of this prophylaxis if this calamity is to be avoided. The mass ofliterature generated in the past few years has produced some confusion as to which of a number of methods of heparin prophylaxis gives the best results. It is the intention of this book to help clarify the situation and thereby resolve the problem by offering a procedure of subcutaneous heparin prophylaxis which is acceptable, safe and simple to administer. Recent technical advances offer the hope of attaining this goal. 7 Acknowledgements I wish to acknowledge with thanks the assistance given me in preparing this book to Miss Viola Johnson, my former secretary, Mrs Mary Coan, the hospital librarian and Dr Alex Silverglade of Riker Laboratories, Inc. , USA.