A lung abscess is a circumscribed area of pus or necrosis in the pulmonary parenchyma. From the point of view of treatment, is necessary to understand whether lung abscesses are primary (abscesses that are not associated with a local structural abnormality; or systemic vulnerability such as an immunecompromising state). In secondary lung abscesses a background predisposing condition plays a part, both in its pathogenesis and in the adequacy of its response to treatment. This chapter addresses the issues involved in the workup of lung abscesses. It addresses the non-infective conditions that can mimic a lung abscess, and provides guidance on uncovering local and systemic predisposing factors that may not be obvious in a given clinical situation. It also discusses the causative microorganisms that are likely in various situations, and the limitations of routine testing in the immunecompromised host.