This chapter focuses on size of various tubes or cannulas. Size of Ryle\'s tube with color codes, size of spinal needle with color codes, size of suction catheter with color codes, size of Foley’s catheter with color codes, venflon cannula size and color code, sizing guidelines are described in the table format. The size of the gauge increases with the external diameter of cannula decreases. Use large diameter cannula for blood transfusion to prevent hemolysis. Complications of cannulation are infiltration, hematoma, air embolism and phlebitis.