This article provides an overview of interactions between charged interfaces across concentrated suspensions of charged nanoparticles or solutions of polyelectrolytes. These systems bear many similarities. We distinguish the likecharged and oppositely charged situations. In the like-charged situation, a layered structure in the proximity of the interface is formed. This structure induces a strongly repulsive energy profile at shorter distances, which originates from a gap that is free of nanoparticles or polyelectrolytes. At larger distances, the profile becomes oscillatory. This energy profile can be quantified with a simple model, which distinguishes the nearfield region and the far-field region. The parameters entering the model show characteristic scaling relations. In the oppositely charged situation, a saturated, tightly bound layer at the interface forms. This layer leads to a charge reversal of the interface and induces a similar layered structure as in the likecharged case.