A computer simulation of the Er 3+ 3-μm crystal laser considering the full rate-equation scheme up to the 4 F 7/2 level has been performed. The influence of the important system parameters on lasing and the interaction of these parameters has been clarified with multiple-parameter variations. Stimulated emission is fed mainly by up-conversion from the lower laser level and in many cases is reduced by the quenching of the lifetime of this level. However, also without up-conversion a set of parameters can be found that allows lasing. Up-conversion from the upper laser level is detrimental to stimulated emission but may be compensated by cross relaxation from the 4 S 3/2 level. For a typical experimental situation we started with the parameters of Er 3+: LiYF 4. In addition, the host materials Y 3 Al 5 O 12 (YAG), YAlO 3, Y 3 Sc 2 Al 3 O 12 (YSGG), and BaY 2 F 8, as well as the possibilities of codoping, are discussed. In view of the consideration of all excited levels up to 4 F 7/2, all lifetimes and branching ratios, ground-state depletion, excited-state absorption, three up-conversion processes as well as their inverse processes, stimulated emission, and a realistic resonator design, this is, to our knowledge, the most detailed investigation of the Er 3+ 3-μm crystal laser performed so far.