For the sake of clarity and coherence, this chapter devoted to the nature of quasi-particles in organic–inorganic halide perovskites will concentrate on lead-based perovskites. The exceptional light emission properties of layered hybrid organic-inorganic perovskite (HOP) thin 202films or crystal structures have been well documented in the literature since the early 1980s (Ishihara et al. 1990; Koutselas et al. 1996; Mitzi et al. 1995). Intense light emission is indeed observed from helium to room temperature. Light absorption experiments have unambiguously revealed the excitonic character of the optical absorption at the electronic band gap edge. Extremely large exciton binding energies, approximately 300 meV (see Figure 7.1 and Ishihara et al. 1990), have been measured as well as bound state resonances for the 1s up to 4s exciton states or for the biexciton and triexciton transitions (Even et al. 2012; Kato et al. 2003; Muljarov et al. 1995; Shimizu et al. 2006). Conversely, until recently little was known about excitons in 3D perovskites. The 50 meV binding energy recorded at 4 K in the early stages has only recently been shown to correspond to a large overestimation of the room temperature value (Even et al. 2014a; Hirasawa et al. 1994).