In this chapter, we review various probing waveform transmission schemes for multiple-input multiple-output (MIMO) radar with co-located antennas. An orthogonal probing waveform set is required to separate the transmitted waveforms at the receiver side to achieve a large virtual array size afforded by the MIMO radar. This increased virtual aperture size provides MIMO radar systems with many advantages, including better spatial resolution, improved parameter identifiability and enhanced performance for ground moving target indication (GMTI) and radar imaging. We discuss several MIMO radar transmission schemes herein, including fast-time code division multiple access (FT-CDMA), frequency-division multiple access (FDMA), time division multiple access (TDMA), randomized TDMA (R-TDMA), Doppler division multiple access (DDMA) and slow-time CDMA (ST-CDMA). The merits and limitations of these transmission schemes will be discussed, and brief examples will be presented for illustration purposes.