**1. Introduction**

The use of nanomaterials as carriers for the administration of drugs and therapeutic agents is gaining increased attention. These nanocarriers are facilely taken up by the cells and are able to deliver the drug to the target site and prevent its rapid clearance or degradation [1]. Although several inorganic (such as iron oxide NPs, noble metal NPs, quantum dots, etc.) and organic (such as liposomes, polymers, dendrimers) nanomaterials have been produced as nanocarriers, each of these classes of nanomaterials has its own set of merits and demerits [2, 3]. Only a few of these nanosized drug carriers have been approved by the US Food and Drug Administration (FDA); though still, they have some limitations [4].

Metal–organic frameworks (MOFs) also referred to as porous coordination polymers (PCPs) are a crystalline class of coordination polymers and were first reported by Bernard F. Hoskins and Richard Robson in 1989 [5, 6]. MOFs are being synthesized in a building block fashion, in which inorganic building units (metal ion vertices or clusters) are interconnected by organic building units (organic linker molecules) by a self-assembly process, to form highly tailorable crystalline materials having pores in the nanometer range [7]. Their unique combination of high porosity, large surface areas, lack of non-accessible bulk volume, a wide range of pore sizes (micro- or mesopores), shapes (cages, channels, etc.) and topologies, tunable and rigid frameworks, easy surface functionalization, and a limitless number of possible combinations of metals and ligands have resulted in a large number of their potential applications [8, 9].

Nanoscale Metal–organic frameworks or Metal–organic framework nanoparticles (NMOFs or MOF NPs), nanoscale counterparts of MOFs are an attractive class of hybrid nanomaterials. These NMOFs not only exhibit the unique features of porous nanomaterials, but they also have benefits over analogous bulk MOFs for a variety of biomedical applications due to their small size. They can offer many advantages over conventional nanocarriers. (i) First, they can be designed to form desired structures with different shapes, sizes and chemical properties allowing for the loading of various therapeutic agents with different functionalities; (ii) next, their large surface area, high porosity, uniform pore size and volume results in high loading efficiency and selective transport; (iii) further, as a result of their somewhat labile metal and ligand coordination bonds, they are intrinsically biodegradable, which prevents their accumulation in the body after their task is achieved; (iv) finally, their surface functionalization by post-synthetic modifications can improve their colloidal stability, thereby prolonging their blood circulation time [10–12]. Thus, the miniaturization of MOFs to NMOFs has resulted in the development of nanomaterials with great potential to be used as drug delivery systems. The structural flexibility (referred to as "breathing") and switchability of MOFs is a unique feature not found in other porous materials [13].

This chapter will give the readers an overview of the use of MOFs and NMOFs as potential drug carriers. In the succeeding sections, the basic composition and structure of these porous frameworks and general synthetic routes adopted for their preparation shall be discussed. Commonly used drug incorporation techniques and characterization methods to verify drug association will also be presented. In the final section, a summary of some of the MOFs and NMOFs reported as carriers and for application in the delivery of therapeutic drugs, biomolecules such as proteins, nucleic acids, carbohydrates, and other active agents employed for light and magnetic field activated therapies, shall be provided.
