This thesis describes a Magneto-Optical Kerr Effect (MOKE) and Magnetic Circular Dichroism (MCD) study of ferromagnetism in III-V magnetic semiconductor heterostructures. This work was initiated due to the much recent interest in spin-related phenomena in semiconductors. The discovery of carrier/light-induced ferromagnetism in InMnAs-based heterostructures has opened up new possibilities for spin-based multifunctional devices that will integrate photonics, magnetics and microelectronics. However, an experimental demonstration of direct ultrafast optical manipulation of spins/magnetic order in this system has not been reported, although it is considered to be a crucial step towards pico- or femto-second operation of information encoded in the spin degree of freedom. In addition, the origin of ferromagnetism is still a subject of debate. To address these issues, we have developed an experimental setup for performing continuous-wave (CW) and time-resolved light-induced MOKE/MCD experiments. Our data clearly demonstrate that magnetic properties, e.g., coercivity and remanent magnetization, can be optically controlled in an ultrafast manner (∼ ps). The experimental results of CW and two-color time-resolved MOKE/MCD measurements are presented. Finally, the mechanism of the ultrafast MOKE response is discussed in terms of transient photo/carrier-induced modifications of exchange interaction and domain wall energy.