This paper reports some of the studies on nonconventional ICF approaches performed at the ICF Physics and Technology Laboratory of the AEEF in Frascati, Italy. Having as reference potential difficulties associated to the conventional central spark ignition (fuel mixing) and to the usual approach to fast ignition by laser (transfer and coupling of the energy pulse, fast electrons energy tuning), we have made explorative work on possible alternatives. The performances of targets ignited near stagnation by pulses of heavy ion beams (HIB) or by macroparticle impact were previously studied. The needed driver energy, the power, and the beam quality requirements, as well as the level of synchronization the implosion and the igniting pulse have been found. More recently, to relax some requirements on the HIB beam parameters set by the previous approach, the injected entropy approach (IE) has been introduced. In this method, the conditions for spark formation are set in the final stages of the implosion, when the spark fuel size is a few times the final size at stagnation (volume a few tens of the final). Energy is injected at this time to set the spark fuel on a high adiabat. In this paper, for illustration and comparison purposes, some relevant results we previously obtained for near-stagnation ignition are first introduced and critically reviewed. The new IE method, after a short analytical introduction, is presented and illustrated by the results of extensive 2-D numerical simulations. The considered cases refer to imploding cylinders of finite length. As required by this approach, one or two opposing beams axially injected additional energy, whereas the acceleration stage of the cylindrical low-entropy implosion was assumed driven by a different driver. Heavy ion beams, soft X-rays (SXR), and laser generated light ion beams were considered as vectors for the entropy injection. Issues related to the feasibility of these generators are discussed. The study was made for various initial conditions leading to different ignition modes and burn propagation. The most recent results on the injected entropy method to the ignition of high gain targets are included.