Abstract: Provided are CD3 binding molecules that specifically bind to CD3, for example monospecific binding molecules that specifically bind to CD3 and multispecific binding molecules (MBMs) that specifically bind to CD3 and a tumor-associated antigen, conjugates comprising the CD3 binding molecules, and pharmaceutical compositions comprising the CD3 binding molecules and conjugates. Provided are methods of using the CD3 binding molecules, conjugates, and pharmaceutical compositions to activate T cells in a subject, for example a subject having a cancer or autoimmune disease. Provided are recombinant host cells engineered to express the CD3 binding molecules and methods of producing the CD3 binding molecules by culturing the host cells under conditions in which the CD3 binding molecules are expressed.