Crystalline nanoporous materials are one of the most important groups of solid-state material with widespread utilization in a host of applications.[1] The successful development and application of these materials will be greatly facilitated through a full understanding of the crystal growth mechanism. This will permit control of properties such as composition, structure, size, morphology, and the presence and form of defects within the crystals. As the product of the crystallisation process is determined by the process parameters themselves, the investigation and understanding of this process is of comparable importance to the correct choice of the reagents.[2] Atomic force microscopy (AFM) is a technique that permits detailed observation of nanometer-sized features on crystal surfaces and unique insight into crystal growth processes through ex situ and in situ studies.[3] Hitherto the application of AFM to the understanding of crystal growth of crystalline nanoporous materials has been limited to conclusions drawn from ex situ studies of crystal surfaces and in situ studies of crystal dissolution.[4] In situ AFM studies of the crystal growth of this type of material are required to present definitive real-time evidence for the mechanism and to provide information on the nature of the fundamental structural units attaching to the crystal surface during growth. The latter can be related to the likely involvement of specific aggregates or secondary building units (SBUs) in the growth solution, one of the central questions in the synthesis of nanoporous materials.[5] Here we present such evidence obtained from the first high-resolution in situ AFM study of the crystal growth of a crystalline nanoporous material, HKUST-1. This information will aid in the understanding of the mechanism of the reaction–crystallization processes that result in the formation of extended datively or covalently bound materials. The copper trimesate Cu3 [(O2C) 3C6H3] 2 (H2O) 3 (HKUST-1)[6] is an important crystalline nanoporous metal–organic framework (MOF) that is receiving considerable interest in a variety of areas including the development of oriented membranes, and gas storage and purification.[7–10] HKUST-1 is assembled from Cu2 (H2O) 2 dimer units and tridentate trimesate (benzene-1, 3, 5-tricarboxylate) groups to form a three-dimensional framework structure containing a threedimensional channel system, with channels of pore size 9.5 (see Figure S1 in the Supporting Information). The high-resolution AFM deflection micrographs of the growing {111} face of HKUST-1 as a function of time are shown in Figure 1 (see Figure S2 in the Supporting Information for additional micrographs). The micrograph taken before injection of the growth solution, Figure 1a, reveals a complex surface topography of the substrate HKUST-1 crystal that contains several growth steps, a number of line defects and other gross defect structures. The crystal face is seen to change drastically after injection of the growth solution with numerous two-dimensional surface nuclei forming all over the crystal surface (Figure 1b). As AFM cannot distinguish between homogenous and heterogenous nucleation, we refer to this process as two-dimensional surface nucleation. The lateral dimensions of the two-dimensional nuclei are smaller at the top of Figure 1 b than at the bottom indicating growth and spreading of the nuclei during the time taken to scan the crystal surface. Surface islands are visible in Figure 1c indicating that the supersaturation has decreased sufficiently to reduce the rate at which fresh two-dimensional surface nuclei form and so the lateral spread of the previously formed nuclei becomes the more dominant growth …