The usage of natural gas in internal combustion engines involves various difficulties, like weak lean-burn ability, low flame speed and ignitability, which demand deep studies for its usage in IC engines. For compressed natural gas (CNG) in SI engines, there is an engine efficiency sacrifice at low loads and high levels of hydrocarbon (HC) and carbon monoxide (CO) emissions which cannot be solved without using after-treatment equipment. This equipment, however, is very expensive. Therefore, an additional fuel can enhance the characteristics of the combustion of natural gas, which can be added in the intake charge. Hydrogen is an effective gas for enhancing the flame rate regarding combustion in an SI-CNG engine, in addition to increasing engine stability. Small amounts of hydrogen improve performance and reduce exhaust emissions. Thus, a number of investigators have carried out research studies on SI engines with different ratios of HCNG. This paper is comprehensive overview of CNG, H2 and HCNG blends. The main topics discussed consist of the combustion fundamentals of natural gas, hydrogen and hydrogen-natural gas mixture. Natural gas and hydrogen usage as fuels and their characteristics have been analysed. The storage of hydrogen and HCNG is still challenging researchers and, therefore, their storage has been taken into consideration. Moreover, a comprehensive review has been performed of HCNG blends in order to understand the effect of hydrogen enriched CNG on performance and the emissions of SI engines. The combustion characteristics of HCNG engines are strongly dependent on the conditions of the engine. The air-fuel ratio, the time of injection, the compression ratio and speed play a major role in blending HCNG in an SI engine and have been discussed in this article.