High-density polyethylene (HDPE) piping systems are increasingly recognized for their exceptional corrosion resistance, flexibility, long-term mechanical durability, leak-free monolithic joints, fast and easy installation, and overall cost-effectiveness across a range of industrial sectors, from municipal water and gas networks to safety class nuclear power plant piping applications. The long-term reliability of these systems hinges critically on the integrity of butt-fusion (BF) joints, which often represent the most vulnerable points under operational stresses and environmental conditions. This review serves a dual purpose, it distils best practices for fusion-condition optimisation and evaluates joint-integrity assessment methods, offering a unified guideline for butt-fusion jointing (BFJ) of HDPE pipes. Fundamental fusion parameters such as fusion temperature, interfacial pressure, and heating/cooling durations that govern polymer chain interdiffusion and resultant joint strength are explained. Key standards are examined for their differing recommendations, underscoring the need for harmonized global guidelines. Standard joint integrity testing methods and recent advancements in mechanical testing, ranging from short-term tensile and impact assessments to long-term creep and slow crack growth (SCG), were critically reviewed. Emerging approaches such as, tapered-waist tensile (TWT) specimens, and hydro-axial tension (HAT) evaluating the full joint thickness, offer steps toward addressing the pronounced limitations of existing test methods revealed. The review concludes by highlighting critical research gaps, including real-time joint monitoring for proactive defect mitigation, adaptive multi-stage pressure protocols, and the development of reliable qualification and testing methods specifically tailored to large-diameter, thick-wall HDPE pipes. Addressing these issues is pivotal for advancing BFJ performance, ultimately fostering globally standardized, next-generation HDPE pipelines with trustworthy reliability in high-stakes engineering applications.