Conduction system pacing (CSP) is a physiological approach to cardiac resynchronization (CRT) where cardiac electrical activation can be restored to normal. The standard form of CRT is Biventricular pacing (BVP), this was introduced in 1994 by Cazeau et al who demonstrated haemodynamic benefit. Multiple large randomized controlled trials have shown mortality and morbidity benefit with BVP. However, some BVP receivers did not benefit, and overall mortality and morbidity rates remained high. Conduction system pacing emerged as an alternative option because it delivers a more physiological form of CRT. His bundle pacing (HBP) was pioneered in 2000 by Deshmukh et al in patients with advanced heart failure. Left bundle branch area pacing (LBBAP) emerged as an alternative modality in 2017. Early reports suggested that pacing parameters were better than HBP and it has can correct more distal conduction abnormalities. Although HBP and LBBAP are both safe and feasible modalities for CRT, it is not known which one is better. This thesis explores the effectiveness of HBP and LBBAP to deliver CRT. In Chapter 3, a within-patient comparison of the two modalities, and compared to BVP is demonstrated. In Chapter 4, multiple non-invasive investigations were performed before CSP. The aim was to identify patient characteristics that predict successful CSP. In Chapter 5, I investigated the impact of septal scar on the success of LBBAP. This is crucial for procedural planning and tool selection. In Chapter 6, I explore variable right ventricular activation patterns associated with different programming of devices with LBBAP lead. I investigate the impact of anodal stimulation on ventricular activation times. This thesis elucidates the effectiveness of two novel CRT modalities targeting the conduction system and compares them to BVP. It provides a framework for patient selection for conduction system pacing, which can be utilized in future clinical trials.