## 1 Introduction  
The usage of composite laminates has become more widespread and attracted the interest of many industries such as marine, automobile and aerospace. The higher strength-to-weight ratio in comparison with metallic alloys helps to reduce the weight of the automobile or of the aircraft, consequently improving the fuel efficiency. The usage of composite laminates can decrease the number of parts in a structure and they may have a longer life cycle compared with metallic components, which reduces the maintenance and replacement costs.  
Currently, the most widely used composite materials are made of thermoset resins, such as epoxy, phenolic and polyester, which demonstrate great mechanical and good thermal properties. Despite these valuable properties, due to their low toughness, they tend to be weak, particularly in the transversal direction when subjected to impact loading [1]. The use of thermoplastic-based laminates can decrease this drawback significantly [2] as their toughness is much higher than thermoset polymers. However, the manufacturing cost is higher for thermoplastic composites and they provide lower stiffness, compared with the reinforced thermosets.  
Fiber-reinforced composites are notch sensitive and lose much of their structural integrity when damaged. Damage can be caused during service and may be introduced by machining of fastener'sholes, stress concentrations near designed cutouts, or accidentally dropping tools on the composites. In-service damage of composite airframes may also result from impact by runway debris, hailstones, bird strike, ground service vehicles, ballistics, etc. In many instances, the damage caused by such impacts may be invisible or barely visible on the surface but can significantly reduce the strength of the composite component. Such damage can cause significant reduction in the compression after impact (CAI) strength, which is a typical measure of the damage tolerance of fiber-reinforced composites. Many factors determine the damage resistance and damage tolerance of fiber-reinforced composites. Among these factors, mechanical properties of fiber and matrix, interface/interpase properties and fiber configurations play important roles in determining impact damage resistance and damage tolerance of composites [3].  
Up to now, various methods have been suggested to improve the interlaminar strength and the impact resistance of composite laminates. Some of these strategies are: Z-pinning [4], tufting [5], 3D weaving [6], stitching [7] and matrix toughening [8; 9]. The last of these methods has attracted the researchers' attention as the others can significantly decrease the in-plane mechanical properties [10; 11]. Matrix toughening can be done by adding micro- or nano-sized fillers into the matrix or by interleaving film, fibers, or particles between the composite layers. In this review paper, the focus will be on the behavior of toughened composite laminates under impact loading using nano fillers (in the form of particle or fiber).  
A search on Scopus made using the keywords: Nano, impact, composite laminate, shows that at least 144 papers have been published in this field (Figure 1). As seen in this figure, about 70% of the papers have been published after 2012.  
This review study is divided into two main parts. In the first part, the effect of different nanofibers types, such as Nylon66 (NY66), carbon, and Polycaprolactone (PCL) on impact response of laminated composites are presented. The effectiveness of each type of nanofiber and their toughening mechanism are also considered. In the second part, laminates toughened by nanoparticles, such as carbon nanotubes and nano-clay, are reviewed. It is shown that some geometrical factors such as nanofibrous  
Figure 1: Indexed papers in Scopus (12 September 2018) using the keywords: nano, impact and composite laminate.  
mat thickness and impact energies affect the efficiency of the toughening mechanism. The nanoparticles considered may have 0D, 1D, 2D, and 3D dimensional reinforcing patterns.