Freeze-in is an alternative mechanism for dark matter (DM) production, where a Feebly Interacting Massive Particle (FIMP) interacts so weakly with the thermal bath that it never reaches thermal equilibrium. Unlike the conventional "thermal freeze-out" mechanism, freeze-in yields are dominated by low temperatures near the FIMP mass and are independent of unknown ultraviolet (UV) physics, such as the reheat temperature after inflation. FIMPs can arise from various models, including string theory compactifications with weak-scale supersymmetry breaking, Dirac neutrino masses, or GUT-scale-suppressed interactions. Experimental signals of freeze-in and FIMPs include the production of new metastable coloured or charged particles at the LHC and alterations to big bang nucleosynthesis (BBN). Freeze-in yields are IR dominated and depend on the interaction strength with the thermal bath. The freeze-in mechanism is the opposite of freeze-out, with freeze-in starting with a negligible DM abundance and increasing it through interactions with the bath. Freeze-in can produce DM that is either the FIMP itself or a lighter particle resulting from FIMP decay. The freeze-in yield is given by $ Y_{FI} \sim \lambda^2 (M_{Pl}/m) $, where $ \lambda $ is the coupling strength and $ m $ is the FIMP mass. Freeze-in and freeze-out share common features, such as the final abundance being determined by initial thermal conditions and IR physics. The freeze-in mechanism is particularly useful for producing DM with masses ranging from superheavy to weak scale, as the final relic density is independent of the FIMP mass. The freeze-in mechanism is implemented in various models, including string theory, extra-dimensional extensions of the Standard Model, and models with Dirac neutrino masses or kinetic mixing. The freeze-in yield is also affected by higher-dimensional operators and UV contributions, which can dominate in certain scenarios. Experimental signals of freeze-in and FIMPs include long-lived LOSP decays at the LHC, reconstructed LOSP properties, enhanced direct and indirect detection signals, and perturbed BBN abundances. The freeze-in mechanism provides a viable alternative to freeze-out for producing DM, with distinct experimental signatures and theoretical implications.Freeze-in is an alternative mechanism for dark matter (DM) production, where a Feebly Interacting Massive Particle (FIMP) interacts so weakly with the thermal bath that it never reaches thermal equilibrium. Unlike the conventional "thermal freeze-out" mechanism, freeze-in yields are dominated by low temperatures near the FIMP mass and are independent of unknown ultraviolet (UV) physics, such as the reheat temperature after inflation. FIMPs can arise from various models, including string theory compactifications with weak-scale supersymmetry breaking, Dirac neutrino masses, or GUT-scale-suppressed interactions. Experimental signals of freeze-in and FIMPs include the production of new metastable coloured or charged particles at the LHC and alterations to big bang nucleosynthesis (BBN). Freeze-in yields are IR dominated and depend on the interaction strength with the thermal bath. The freeze-in mechanism is the opposite of freeze-out, with freeze-in starting with a negligible DM abundance and increasing it through interactions with the bath. Freeze-in can produce DM that is either the FIMP itself or a lighter particle resulting from FIMP decay. The freeze-in yield is given by $ Y_{FI} \sim \lambda^2 (M_{Pl}/m) $, where $ \lambda $ is the coupling strength and $ m $ is the FIMP mass. Freeze-in and freeze-out share common features, such as the final abundance being determined by initial thermal conditions and IR physics. The freeze-in mechanism is particularly useful for producing DM with masses ranging from superheavy to weak scale, as the final relic density is independent of the FIMP mass. The freeze-in mechanism is implemented in various models, including string theory, extra-dimensional extensions of the Standard Model, and models with Dirac neutrino masses or kinetic mixing. The freeze-in yield is also affected by higher-dimensional operators and UV contributions, which can dominate in certain scenarios. Experimental signals of freeze-in and FIMPs include long-lived LOSP decays at the LHC, reconstructed LOSP properties, enhanced direct and indirect detection signals, and perturbed BBN abundances. The freeze-in mechanism provides a viable alternative to freeze-out for producing DM, with distinct experimental signatures and theoretical implications.