There are four known models of black hole accretion disks, of which the Shapiro-Lightman-Eardley (SLE) disk is unstable, the slim disk has not been studied in detail, and quite successful are the Shakura-Sunyaev disk (SSD) that was first established and the advection-dominated accretion flow (ADAF) that has received much attention in recent years. It seems that SSD and ADAF are adequate to the outer and the inner region of the black hole accretion flow, respectively, thus their combination, i.e. the ADAF+SSD model is likely to be able to give a complete description of black hole accretion flows, although there are still many open questions regarding this combining model.