Wake-up acute ischemic stroke is medical emergencies as in reality there is a narrow window for acute reperfusion therapy with iv thrombolysis in this group. There is a significant amount of patients who presents with wake-up acute ischemic stroke reaching around 30%. If no appropriate diagnostic tools are used clinician is likely to miss opportunities for IV thrombolysis leading to increased morbidity leading to acute stroke patients. Among the available imaging techniques CT brain, CT perfusion, and MR diffusion perfusion, MRI diffusion, and FLAIR (fluid-attenuated inversion recovery) mismatch techniques can be used to determine and quantify viable tissue (penumbra) in this wake-up stroke group. In this article, we review the current recommendations and best available imaging techniques and how they can be used in the identification of penumbra for acute reperfusion therapies in acute ischemic stroke which has presentation as wake-up stroke.