The use of antibodies (Abs) and enzymes in diagnostic assays is widely accepted and, to date, represents the gold standard in terms of sensitivity and affinity. Abs are routinely used in many diagnostic assays but they suffer from short shelf-life, high costs of manufacturing and relatively poor stability, especially at extremes of temperature and pH. The use of molecularly imprinted polymer (MIP) nanoparticles can overcome these problems. Compared with Abs, the synthesis of MIPs is simpler and more cost-effective and, moreover, does not require involvement of animals. In addition, MIPs show high stability and excellent mechanical properties, and can be prepared virtually for any target. NanoMIPs, sometimes called ″plastic antibodies″, are nanostructured polymer particles capable of selectively recognising the said target. Thanks to their size, they represent a viable alternative to Abs, as demonstrated by their recent application in several diagnostic fields. Herein, we review the most common synthetic approaches in the manufacture of nanoMIPs, together with some recent examples of the use of nanoMIPs in diagnostics, in particular within sensors, assays and for imaging purposes.