Lymphedema, which can be a debilitating sequela to breast cancer treatment, is characterized by an abnormal accumulation of lymph in the arm, shoulder, breast, or thoracic area. It may appear gradually or suddenly, and although it usually develops within three years of a breast cancer diagnosis, it can arise much later; survivors remain at lifetime risk. The condition can cause physical discomfort and pain, impaired function, and emotional distress. It's imperative that survivors' risk of lymphedema be reduced and that those who develop it receive help to manage it. Part 1 of this two-part article describes post-breast cancer lymphedema and discusses its diagnosis and measurement. Part 2 (next month) will discuss risk reduction, treatment, and implications for nurses.