Internationalism has been a prominent theme in the transatlantic history of social reform and may pioneering social workers championed the causes of peace, war relief, human rights, and international avenues for dispute resolution. For some, internationalism was inseparable from social welfare. Jane Addams in the United States, Alice Masaryk of Czechoslovakia, Alice Salomon of Germany were central figures during the early 20th century in transatlantic campaigns for social work. This article draws from letter, autobiographies, and published works of these three pioneers to reconstruct their social networks and to examine their contribution to the transatlantic transfer of knowledge. International diffusion of thought and practice has high salience for a profession embracing change in the 21st century.