Medical needles are sterile, single-use stainless steel cannulas with a beveled tip for piercing skin/vessels, sized by gauge (higher number = thinner diameter: 14G 2.1mm for blood transfusion to 33G 0.2mm for insulin) and length (¼-3½"). Hypodermic (IM 21-23G 1-1½", SC 25-27G ⅜-⅝", ID 26-27G ⅜") for injections/blood draws; IV catheters (14-26G color-coded); spinal (22-25G Quincke/Whitacre pencil-point for CSF); epidural (Tuohy 16-18G curved); butterfly/scalp vein (21-25G winged for peds); biopsy/bone marrow (thick trocars). Luer lock hubs ensure secure syringe fit; safety variants retract to prevent sticks.