Systemic lupus erythematosus (SLE) is a chronic inflammatory autoimmune disease, as yet of unknown aetiology, with diverse clinical manifestations and a variable course and prognosis. The diagnosis is based on recognizing the overall pattern of clinical and laboratory abnormalities. However, even today, there is often a significant delay between onset of symptoms and diagnosis. Over the last two decades there has been great progress in identifying the profile of antinuclear antibodies that characterizes SLE, and in this chapter we describe how serological techniques can be important tools for the clinician in the early diagnosis of this disorder. Also described is the lupus band test, which has rather fallen out of favour as a diagnostic tool but which can still provide valuable evidence for the diagnosis in patients in whom the clinical and serological features are inconclusive. Nevertheless, because the presentation of lupus is protean, and the early manifestations are often non-specific, SLE can still be easily confused with a wide range of other conditions. Here, we describe some of the common clinical conundrums encountered in patients referred to the Lupus Clinic to 'rule out lupus', providing a framework for diagnosis. Finally, the chapter considers the major problem that clinicians who treat patients with SLE frequently face in distinguishing between a flare of lupus and infection. Diagnosis of SLE is still a great clinical challenge, and while it is important to recognize patients with potentially aggressive disease and treat them appropriately at an early stage it is also important to be able to recognize patients with potentially benign disease and avoid over-treatment.