Crop production and its sustainability are of the major concerns handled in agenda of the plant researchers and policy makers. The biotic and abiotic stresses of the environment are likely to increase alongside the ever-growing human population. Remarkable nanotechnological attempts have been made to reduce the pressure on plants, as evidenced with bolstered plant growth and enhanced productivity. Over the last decade, nanostructured materials have gained tremendous interest in this context. In this chapter, we have focused on our interest in carbon nanomaterials. The use of carbon nanomaterials in plant science is very limited, at around 2%, compared to the physical sciences, especially materials science. There are two sides to the story of carbon nanomaterials, according to the reports. First, we focused on the positive effects of carbon nanomaterials on plant redox regulation and its manifestations in plant growth performance under adverse environmental conditions. Since the term “toxicity” was very much in evidence in most of the documents, we looked at the issue of toxicity. The hotspots with respect to the toxicity and mechanism of the carbon nanomaterials were as oxidative stress, cell death, and cell membrane damage. Such observed endpoints are manifestations of material concentrations. In this respect, phyto-toxicity has been further discussed in terms of the phenomenon of hormesis or a biphasic curve. However, it is easy to see from the reports available that our knowledge of the mechanisms of carbon nanomaterials and plant interactions is still in its infancy. In the conclusion section, we have suggested some outstanding questions that need to be addressed to further our knowledge of the mechanisms of carbon nanomaterials in plant systems.