The invention provides methods and systems for introducing a catheter into a blood vessel. A catheter set comprising a pre-loaded stylet in a catheter is connectable to a guidewire. The guidewire may be introduced into the patient through a hollow needle. The stylet can then be connected to the guidewire, and the catheter advanced over the stylet and the guidewire into the access site and into position in the blood vessel of the patient. The invention reduces the time and placement steps required to position a catheter in a vessel and lowers the associated procedural costs. The invention reduces the frequency and severity of complications generally associated with introducing a catheter, thereby improving patient outcomes and lowering healthcare costs.