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All parts of the pancreas lie in the retroperitoneal space, except the distal end of the tail of the pancreas, which reaches the hilum of the spleen within the lienorenal ligament.
The head of the pancreas is the flattened enlarged portion that rests within the C-curve of the duodenum. The most inferior portion of the head is the uncinate process, which "hooks" to the left and lies posterior to the superior mesenteric artery and vein.
Links and References:
Grant's: 2.64A
Netter: 280
Rohen/Yokochi: 278, 279