If you are experiencing pain, swelling or lumps in your armpit (referred to as the axilla), the quickest, and harmless way to diagnose this issue would be to get an ultrasound. The armpit is composed of vascular structures such as the artery and vein, nerves and lymph nodes as well as the overlying fat, occasionally accessory breast tissue, skin and subcutaneous glands. Ultrasound is routinely used for the assessment of the axilla to assess lumps and pain, including in the use of assessing lymph nodes for enlargement and potential involvement in cancer. Here is an overview of what may be causing lumps and nodules in your armpit, when it’s a good idea to see a doctor, and what an ultrasound procedure involves.