DLL4 binding proteins are described herein, including antibodies, CDR-grafted antibodies, humanized antibodies, and DLL4 binding fragments thereof, proteins that bind DLL4 with high affinity, and DLL4 binding proteins that neutralize DLL4 and/or VEGF activity. The DLL4 binding proteins are useful for treating or preventing cancers and tumors and especially for treating or preventing tumor angiogenesis.