Digital television becomes more and more important in consumer electronics. De-interlacing technology is a key component of digital television that increases the vertical sampling density of each field. However, the conventional sampling rate conversion theory cannot be generally applied. Due to the above problem, various de-interlacing algorithms have been proposed. In this paper, we present an overview of the most popular de-interlacing techniques reported previously. In particular, the common motion estimation algorithms for interlaced video are introduced in detail. We will give some suggestions for related research in this paper.