The importance of wettability in reservoir evaluation and dynamics in shale is gaining increasing attention. Wettability is also a key consideration in the strategy development of enhanced oil recovery (EOR) in unconventional reservoirs. However, the determination of shale wettability is often elusive, and an understanding still remains incomplete. Several commonly applied assumptions and methods for evaluating shale wettability are considered inaccurate or problematic. In this work, important clarifications about shale wettability and the methods of measurement or evaluation are made. Wettability is studied for six shale samples from Eagle Ford and Wolfcamp Shale formations with increasing thermal maturity using an integrated imbibition and imaging method. Wettability was evaluated based on the results of water-oil displacement via spontaneous imbibition and the dominant pore type in the sample. Wettability of the samples is ranged from strong water-wet (SW) to oil-wet and has a general trend of becoming less water-wet (or more oil-wet) with increasing thermal maturity (Ro value from ~0.45 to 1.4%). A new hypothesis on shale wettability transformation from the original water-wet status is proposed based on the results. This new hypothesis emphasizes the evaluation of shale wettability under a dynamic context of oil-water displacement and oil aging history, and shale wettability is a result of oil-water-rock interaction through the geological time frame. Enhanced oil mobility caused by increasing thermal maturity is the main drive of oil imbibition, whereas pore type and pore size also play an important role in oil-water displacement and consequently wettability transformation. The ease of wettability transformation of the pore system in shale is in the order of calcite > quartz, dolomite >> clay. Pores with mixed boundaries of different minerals fall in between. Other geological factors [e.g., total organic carbon (TOC) and pore pressure] also affect oil imbibition and thus wettability. Important implications of shale wettability on water and oil saturation and on improved oil recovery are also discussed.