In our semiclassical treatment of the laser in Chaps. 8-12, we “ Fourieranalyzed” the electric field. This frequency-domain method has much to commend it: explicit expressions for the field are often obtained, the calcu­ lations generally do not require much computer time, and considerable insight can be gained into the nature of the atom-field interaction which takes place in the laser. There are also shortcomings, however, to the frequency-domain picture. The number of modes which can be treated easily is small, perhaps as few as ten. In general, the mode amplitudes and phases cannot vary ap­ preciably in the atomic lifetimes (the total field, of course, can vary). Further­ more the strong-signal theories are rather complicated.