Hydrogen can easily diffuse in metals, significantly altering elastic and plastic properties. These effects have long standing engineering consequences (e.g., hydrogen embrittlement susceptibility in alloys), and still draw significant attention due to present energy-related challenges and potential transition to hydrogen economy. From a physical perspective, the underlying mechanisms of these effects are interesting. Depending on the metal, and its microstructure, similar levels of hydrogen can trigger different micro-mechanisms, the nature of some of which are highly-debated. In multi-phase metals, multiple mechanisms can be activated simultaneously, further increasing the complexity. In this talk, I will be introducing several new in-situ characterization techniques developed in-house that shed more light on the nature of these mechanisms. These insights provide new guidelines for the design of new alloys, as will be discussed.