Cardiac catheterization is an important diagnostic procedure in cardiology and more recently is being used as an effective and less invasive method of performing interventions, replacing cardiac surgery for a number of indications. However, developments in other areas of cardiovascular imaging (echocardiography, nuclear cardiology, and cardiovascular MR) have also affected the practice of cardiac catheterization over time. For example, in many patients with congenital heart disease, diagnostic cardiac catheterization is now reserved for problem solving. There is increasing concern about the long-term health effects of the X-ray dose following cardiac catheterization. Furthermore, there is also a significant risk from X-ray exposure to the staff in the catheter laboratory during these procedures despite the use of protective shields. Another shortcoming of performing cardiac catheterization under X-ray is the poor contrast of soft tissues such as the heart and great vessels. When positioning guide wires, catheters, balloons, and interventional devices, it is not possible to visualize the relevant structures. The operator has to rely on mental images of the anatomy from previous experience, or on contrast angiographic images acquired earlier in the procedure. This leads to some difficulty and a degree of trial and error, which not only prolongs the procedure, but also has the small risk of perforating the heart or great vessels or other complications. The lack of visualization of the relevant anatomy is particularly a handicap in interventional cardiac catheterization procedures and RF ablation of arrhythmias. Performing cardiac catheterisation procedures under MRI guidance is being developed to address these issues. The first clinical indication of MRI guided cardiac catheterisation has been for diagnostic cardiac catheterization procedures and particularly for measurement of pulmonary vascular resistance (Razavi et al. 3003). For this MR guided cardiac catheterization has been used to acquire simultaneous measurement of invasive pressures and MR flow data. The results were compared with conventional PVR measurements acquired using the Fick calculation, performed during the same procedure. In this study, we demonstrated moderate/good agreement between Fick- and phase contrast-derived PVR at baseline(Muthurangu et al. 2004). However, in the presence of nitric oxide, used to assess pulmonary vasoreactivity, there was less agreement between the two methods. In the presence of 100% oxygen and nitric oxide, there was not only worsening agreement, but also a large bias. The Fick principle is known to be inaccurate and imprecise in the presence of high pulmonary blood flow and high concentrations of oxygen (Hillis et al. 1985). We also demonstrated the in vitro accuracy and precision of phase-contrast MR in our facility using a flow phantom that replicates in vivo conditions more closely than previous studies (Muthurangu et al. 2004). In addition, the oxygen content of pulmonary arterial blood at 100% oxygen is similar to the oxygen content of aortic blood at baseline, where it has been shown that phase-contrast MR is accurate (Beerbaum et al. 2001). We therefore believe that the worsening agreement between the two methods in response to pulmonary vasodilatation is due to errors in the Fick method rather than phase-contrast MR, and that the Fick method underestimates PVR in the presence of 100% oxygen and 20 ppm of NO. This has important implications for patient management, as response to vasodilators is integral to the assessment of patients with pulmonary hypertension. This technique allows more accurate method of PVR quantification, leading to better management of patients with pulmonary hypertension(Pushparajah et al 2015). MR-guided interventional cardiac catheterization has also been performed on few patients: including balloon dilation of pulmonary valve stenosis and aortic coarctation (Tzifa et al 2010). However these procedures have not become routine because of the lack of MR-compatible catheters and devices. In all cases, MR imaging was helpful at the beginning of the procedure for planning the intervention, and at the end of the procedure for evaluating the outcome. However during the procedure the use of MR compatible guide wire with limited mechanical properties was an issue which should be addressed as new MR guide wires with better mechanical properties become available. We have performed a number of clinical cases of electrophysiological study and RF ablation of atrial flutter under MR guidance (Chubb et al 2015). These have used active tracking of electrophysiology catheters (IMRICOR Inc USA) and an MR Electrophysiology interventional research platform called iSuit (Philips Healthcare Netherlands). Again this has shown feasibility and safety but mechanical properties of catheters need to be improved to allow more widespread usage. New generations of catheters are being developed which should address this issue.