This review summarizes the current knowledge on the α″ martensite transformation in metastable β Ti alloys under different stress state conditions gained using various in situ techniques that range from the meso- to the nanoscale. Compared to ex situ observations, in situ observation can be conducted on a single region while changing external stimuli, and are therefore capable of elucidating the interim microstructural evolution. The strengths and shortcomings of different techniques to give insight into the martensitic transformation are identified. Both the formation of α″ martensite and its reversion on load removal are addressed. The phenomena of α″ martensite formation including preferential nucleation sites, its growth, interactions, and variant selection are described. The review also discusses the details of the macro- and micro-level mechanical behavior caused by the phase transformation and highlights the research questions for further investigation.