For decades, xenografts using well-established human tumor cell lines have been the most commonly used models to study human cancers in mice. More recently, human tumors implanted directly into immunodeficient mice have become increasingly popular as evidence accrues that they more accurately recapitulate features of patient tumors. Here we describe our protocols for the orthotopic and heterotopic implantation of pancreatic cancer cell lines and freshly isolated patient tumors into immunodeficient mice. We also describe procedures for the digestion of tumors into single-cell suspensions for the isolation of subpopulations of tumor cells. Orthotopic or heterotopic implantation of established cell lines requires 1–2 h, with 1-cm tumors arising after 2–5 weeks. Engraftment of patient tumor samples takes ∼2 h and growth of palpable tumor requires ∼14 weeks. Once established, direct xenograft tumors require 2 and 5 h for heterotopic and orthotopic implantation, respectively, and 5–6 weeks for palpable tumor growth.