This disclosure provides tools and implant techniques utilizing such tools to gain access to and implant a medical device, such as a medical electrical lead, within extravascular spaces. In one example, this disclosure provides a tool for creating a sub-sternal tunnel in a patient. The tool comprises a relatively straight guide member extending from a first end thereof to a second end thereof, a tunneling member extending from a first end thereof to a tip thereof, the tunneling member extending alongside and coplanar with the guide member, the first end of the tunneling member and the first end of the guide member being joined together, and a handle coupled to the guide member.