Over the past few years, colloidal nanoparticles have started to be investigated for their optical properties in the mid-infrared, past 3 microns. Research on detector application has led to background limited detection and fast video imaging at 5 microns. With further development, one could imagine that these new materials could vastly reduce the costs of infrared technology and this would lead to a trove of new applications for infrared imaging into our daily lives. This article reviews the progress regarding the optical, transport and photodetection properties of thin film based on these materials, and the three different ways by which infrared resonances have been realized with colloidal nanoparticles: interband absorption with small gap semiconductor quantum dots, intraband absorption in lightly doped quantum dots, and plasmonic resonances in heavily doped nanocrystals.