Day by day, counterfeiting is growing as a global problem that has terrible effects on companies, the government, the financial system, public health, and safety [[1], [2], [3], [4]]. Also, counterfeiting has created a threat to industries such as banks and currency, cosmetics, pharmaceuticals, electronics, and packaging. Thus, anti-counterfeiting technology has been widely applied in banknotes, trademarks, security, diplomas, certificates, medicine, etc., to combat counterfeiters [1,[5], [6], [7]]. In order to impede and identify duplication, anti-counterfeiting tags with high density, security, stability, storage ability, easily decodable, and suitable for mass production are urgently required. In fact, various kinds of covert tags or marks are being employed to create unique patterns using color coding or photonic structures that resemble unclonable fingerprints and use them as essential security components to safeguard valuable articles from counterfeiters [4,[8], [9], [10], [11], [12], [13]]. However, these are easily reproducible by counterfeiters because of their simple decoding mechanism, easy fabrication process, and low complexity. But tags with high complexity and security are of high cost, which restricts their use in consumer products. Therefore, it is necessary to create low cost, more secure, anti-counterfeiting technology that can't be reproduced.

During the past few years, fluorescent patterns based on lanthanide doped nanomaterials have gained much interest and developed for anti-counterfeiting applications. This review sheds light on fluorescent security tags that were developed with either single or multi-mode excitation of both lanthanides doped downconversion/downshifting or upconversion nanomaterials. Additionally, it includes a brief overview of other luminescent materials, such as quantum dots (both semiconductor and carbon dots) and metal-organic frameworks, for comparative purposes. While several excellent reviews on lanthanide-doped nanomaterials have been published in the past decade [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]], to the best of our knowledge, no review has specifically addressed their single or multicolor downshifting or upconversion emissions for anti-counterfeiting applications. This review briefly discussed various anti-counterfeiting technology and diverse printing methods that have been employed in anti-counterfeiting field. The potential legal challenges that company encounter when enforcing new anti-counterfeiting measures are also considered. A brief outline of the review has been shown in Scheme 1.