Although the successful treatment of critically ill patients often depends on access to the large veins close to the heart, the indications for this procedure should be strictly limited on account of the attendant risks and potential complications. From the large variety of cannulas, catheters and techniques available for this purpose those systems should be chosen that are simple and safe to use. Seldinger's technique is the method of choice for elective puncture under optimum conditions. If the time factor is important, closed complete systems should be employed. Aseptic puncture and nursing technique is essential to prevent the most common late complication-septicaemia. Radiological verification of the siting of the catheter is important. The various access routes to the central veins carry differing success rate and they also vary in respect of the incidence of complications. This should be borne in mind when deciding on a particular technique. On the other hand, the choice also depends on such factors as technical skill of the physician, the quality of nursing and the expected duration of hospital stay. The peripheral veins of the arm are the safest access for the less experienced physician and for short-lasting catheterization; otherwise the internal jugular vein is the preferred site followed by the subclavian vein. Even if the physician prefers a particular access route he should be familiar with all types of technique since critically ill patients are liable to require changes of the catheter. There is no single technique that will cover all contingencies.