This thesis is a report of the experimental investigation of mode locked optically pumped vertical-external-cavity surface-emitting semiconductor lasers (OP-VECSEL). Mode locked VECSELs are a promising laser technology with transform limited pulses as short as 448fs being generated directly from the laser oscillator. Mode locked VECSELs have the advantages of being; high repetition rate, compact, low cost, robust, power scalable to high output power and near turn key operation. Here, I report the first characterisation of the timing jitter and amplitude jitter of a mode locked VECSEL producing picosecond pulses. A mode locked VECSEL has also been locked to an external electrical oscillator, demonstrating the first active stabilisation of the timing jitter. I also report the first application of a mode locked VECSEL as a laser used to generate broadband THz radiation. The laser dynamics during the mode-locking buildup time of the laser has also been investigated to gain an understanding of the pulse formation mechanisms and pulse duration limiting processes. Finally multiple pulse operation has been observed for the first time and has been characterised.