The invention provides marker delivery devices, assemblies, and methods. Assemblies embodying features of the invention include marker delivery devices having a delivery tube with an orifice, markers, and a plug releasably disposed in the tube occluding the orifice. The plug prevents markers from passing through the orifice before the marker is to be placed at a desired location within a patient's body, prevents ingress of tissue into the bore of the tube, and allows markers to pass out of the orifice when marker delivery is desired. Guidance of a delivery tube to a desired location within a patient's body may include the use of an imaging device, with or without the use of a guide cannula. The plug may itself serve as a marker, and may be the sole marker. Markers may be detectable by ultrasound, X-ray, magnetic resonance imaging, and other imaging devices, and may include bioactive elements.