This article presents different channel estimation techniques in MIMO-OFDM. The grouping of MIMO and OFDM increases the bit rate as well as capacity. A major challenge in the MIMO-OFDM system is the retrieval of channel state information (CSI) correctly and the synchronization between the transmitter end and receiver end. The CSI is retrieved by the help of various types of estimation algorithms such as training based, blind and half blind channel estimation. This article focuses the fundamental introduction of OFDM, MIMO-OFDM system and explains diverse channel estimation algorithms, optimization techniques and their exploitation in MIMO systems for fourth generation (4G) wireless mobile communication systems.