This book presents an introduction to biohydrogen production and the recent advances and developments of the cleanest biofuel produced from bioresources. Biohydrogen has the highest energy content relative to weight and burns cleanly – generating just water. It is the best choice for fuel cells, where it generates electricity directly, in its reaction with oxygen. Biohydrogen occurs naturally as part of digestive gases from mammals and can be produced in specially designed anaerobic biodigesters, or through photocatalysis with microalgae. The gas is also easy to purify and use.  The economic production of biohydrogen is still full of challenges: From the efficient and rapid conversion of the substrate to storage, transportation, and safe use, there are several aspects that need to be developed. Research in this field is addressing the issue of efficient large-scale production from several directions: Substrate pretreatment to enhance digestibility, metabolic networks analysis, microbial diversity and succession to highlight constraints in production, bioreactor, and downstream design to improve throughput and reduce costs, to name a few. The ideas and technologies presented in this book contribute to achieving the UN Sustainable Development Goal 7: Affordable and Clean Energy. The book is written for researchers and students interested in biorefinery and biofuel technologies.