Efficient (> 50%) large aperture (> 10 cm), high energy (> 1 kJ) harmonic conversion has been demonstrated with short pulse (< 5 ns) lasers, notably atomic iodine (1315 nm) and Nd:glass (1064 and 1053 nm). Efficient harmonic conversion utilizes phase matching techniques within selected birefringent crystal media. Considerable interest has arisen to apply these techniques to CW-pumped lasers to enhance their wavelength versatility. When phase matching techniques are applied directly to either long pulse or CW lasers, serious problems arise owing to thermal detuning and crystal damage. Promising approaches using thin harmonic conversion crystals for efficient conversion of quasi-CW lasers are described.