An insertion tool for inserting a catheter into a patient's body is disclosed. The insertion tool unifies needle insertion, guidewire advancement, and catheter insertion in a single device. In one embodiment, the insertion tool comprises a housing in which at least a portion of the catheter is initially disposed, a hollow needle distally extending from the housing with at least a portion of the catheter pre-disposed over the needle, and a guidewire pre-disposed within the needle. A guidewire advancement assembly is also included for selectively advancing the guidewire distally past a distal end of the needle in preparation for distal advancement of the catheter. In one embodiment a catheter advancement assembly is also included for selectively advancing the catheter into the patient. Each advancement assembly can include a slide or other actuator that enables a user to selectively advance the desired component. Guidewire and catheter locking systems are also disclosed.