In this paper Ti (T40) is deformed by interrupted in situ tensile tests. Twinning activation and evolution are evidenced by scanning electron microscopy/electron backscatter diffraction and analyzed by examining their strain accommodation in the frame of all the potential accommodative systems in their neighborhoods. It is revealed that the formation of twins results from the local incompatible strains produced by dislocation slips in grain boundary areas. Either high positive Schmid factor and low positive Schmid factor twins or even negative Schmid factor twins could be activated. Twins are always accompanied by certain accommodative deformation (twinning and slip) in their immediate neighborhoods. Accommodative slips and twins have different impacts on the further development of the initiating twins. Twins accommodated by twins and possessing high Schmid factors have a higher capacity to propagate and repeatedly nucleate; whereas twins accommodated by slips and possessing low Schmid factors have less power and are static. Twins can be permanent or transient depending on their Schmid factors. For those with positive Schmid factors, their strain contributions are consistent with the macroscopic strains, thus they are stable, whereas those with negative Schmid factors are unstable. The formation of such twins is due to the unbalanced local constraints induced by high Schmid factor deformation that generate the so-called back-stress. These twins make opposite contributions to the macroscopic strains. With deformation, the back-stress could be accommodated by local high Schmid factor compatible deformation. The crystal of the twinned part has to make a positive contribution in accordance with the macroscopic strains to be compatible with its surroundings, which may give rise to the activation of detwinning.