An investigation of thermal lensing effects in longitudinally diode-pumped Nd:YVO4 lasers is made in detail both numerically and experimentally. A finite-element model is used to theoretically simulate thermal lensing in various Nd:YVO4 crystals and pumping scenarios. The predicted values for thermally induced lens strengths are compared with experimentally observed results. We find that these two sets of results are in broad agreement, provided that the spatial profile of the pump-induced heat generation is accurately modeled. The saturation of the pump absorption is found to be of particular importance in producing an accurate prediction of the thermal lens strength, and a composite structure of the crystals is desirable to relieve the thermal lensing.