The ESPEN Guidelines on Parenteral Nutrition (PN) reflect current scientific knowledge in the field of clinical nutrition in adults. They summarize the indications for PN and its anticipated outcomes in respect of the underlying disease, nutritional status and quality of life. They are companion documents to the ESPEN Guidelines on Enteral Nutrition and follow the same general format. They address the influence of the underlying disease on the patient's nutritional status, and that of malnutrition on the outcome of the disease. Contraindications to and complications of PN are considered, together with comparative analyses of the roles of the parenteral and enteral routes in different illness states. The quality and strength of the supporting literature has been graded according to the criteria of the Scottish Intercollegiate Guidelines Network (SIGN) and the Agency for Health Care Policy and Research. Hence, meta-analysis of randomised clinical trials (level of evidence Ia) or at least one randomised clinical trial (level of evidence Ib) translate to a Grade A recommendation. Levels of evidence IIa, IIb and III are attributed respectively to: at least one well-designed controlled trial without randomisation; at least one other type of well-designed, quasi-experimental study; or well-designed non-experimental descriptive studies such as comparative studies, correlation studies, case-control studies; each of these sustains a Grade B recommendation. Grade C recommendations reflect expert opinion and/or the clinical experience of respected authorities (level of evidence IV). Each of the 11 sets of PN Guidelines was devised by an international working group, the total faculty comprising no fewer than 87 experts from 16 European/Mediterranean countries, each group's contributions being co-ordinated by a designated chairman. Once each guideline had been approved by all the members of the relevant working group, this version was reviewed by at least two independent external reviewers (one selected from ESPEN's Education and Clinical Practice Committee, and at least one from outside the ESPEN committee structure). Following this review each guideline was hosted in draft form on the public pages of the ESPEN website for at least one month to permit the receipt of comments or suggestions from any interested party. At this point the Guidelines were reviewed and revised again by the original working group chairman and submitted to the Clinical Nutrition editorial process. At least 3 further reviewers were selected by the Journal's editorial office for each guideline, in line with the normal selection process. Final revisions were performed by the Chairmen of the working groups, and by ourselves as commissioning editors of the whole project. More than 300 evidence-based recommendations are now presented. Fewer than one sixth of the recommendations are Grade A, and disappointingly, but unsurprisingly, more than 50% are Grade C. The need for more and better controlled trials in the field remains apparent.