## 1 Introduction  
Over the past 25 years, optical coherence tomography (OCT) has rapidly developed and spread into many fields of biomedical research thanks to its ability to capture label-free structural contrast deep inside a tissue [1]. Over the years, OCT has been developed into numerous different modalities that emphasize the 3D interplay between tissue microstructures and various specific complementary contrasts [2; 3; 4; 5].  
Full-field optical coherence tomography (FF-OCT) is a less familiar OCT-derived technology [6; 7]. FF-OCT differs from standard OCT by acquiring 2D _en face_ images in a widefield configuration at a given depth in a tissue. To record a 3D tomogram, the sample is further scanned in depth, instead of acquiring series of A lines as in regular OCT. Such a configuration offers the possibility to use high numerical aperture (NA) microscope objectives, without limiting the exploration depth, and can therefore access submicrometric and subcellular resolution.  
FF-OCT has been successfully applied for non-invasive label-free  ex vivo pathology assessment in various tissues [8; 9; 10; 11; 12; 13; 14; 15]. Thanks to its isotropic micrometric resolution, FF-OCT can often be compared to histology slices. In contrast to histology, FF-OCT does not require time for sample fixation, slicing, or labeling, and an histology-like image can be produced within a few minutes after tissue extraction. It also helps to reduce fixation and slicing artifacts, as FF-OCT does not necessarily require physical slicing, and can image in depth, eventually far from the incision edges for a biopsy.  
However, FF-OCT may suffer from its lack of specificity, and sometimes fails to equal the diagnostic capacity of gold standard histology. Yet, FF-OCT detection scores are above 80% for most pathologies in both sensitivity and specificity [14; 16], and [PERSON] and colleagues could even report a 100% correct identification of tumors in ex vivo kidneys [17], obtaining similar results to histology, even though the tumor subtyping is still not 100%. In order to increase diagnosis scores, FF-OCT has therefore expanded to multimodal systems combining FF-OCT structural contrast with other pathology biomarkers inside a single microscope. The interest of such multimodal systems is to reduce the ambiguity in structure identification that can arise from sequential imaging with two different microscopes, and to reduce operation time. Some FF-OCT multimodal systems have been reviewed in the recently published _Handbook of Full-Field Optical Coherence Microscopy_, edited by [PERSON] [16]. We can non-exhaustively mention the multimodal combinations of FF-OCT with fluorescence [18], photothermal imaging [19], elastography [20], polarization [21], multispectral and hyperspectral imaging [22; 23; 24]. Most of these multimodal systems have been inspired by similar systems developed in standard OCT, but they needed to be adapted and redesigned to fit FF-OCT's particular configuration.  
In this paper, our goal was not to exhaustively describe these multimodal systems, or to systematically review all the potential fields in which multimodal FF-OCT systems can be of interest. Instead, we will start from the need to increase the FF-OCT's diagnostic ability up to equal that of gold standard histology to describe three different multimodal systems that we are currently developing or improving in our lab. After a brief description of FF-OCT, and the optical systems that we commonly use, we describe and emphasize the differences between FF-OCT and histology contrasts, to better apprehend what contrasts would be optimal to add to FF-OCT. Then, we will describe our recent development related to multimodal systems combining FF-OCT with elastography, with cellular metabolism, and with fluorescence, and how these multimodal systems could increase the diagnosis performance of FF-OCT.