## 1 Introduction  
To celebrate the 25 th anniversary of the introduction of OCT, the special feature issue entitled \"Development and Application of Optical Coherence Tomography (OCT)\" had been initiated. OCT originated from low coherence interferometry [1] and was adapted for tomographic imaging in 1991 [2]. In OCT, broad bandwidth light is used in order to produce cross-sectional images of turbid and translucent samples with high axial resolution (in the order of a few \(\upmu\)m). Thereby, the imaging speed of OCT can be as high as several millions of depth scans (A-scans) per second, which allows for volumetric investigations of dynamic processes [3]. Nowadays, OCT can be regarded as a well-established technique that offers various applications in different fields. Some of these applications and latest developments are covered in the special feature issue, consisting of three overview papers and nine research papers. These are outlined below in the order of their appearance in the feature issue. The high imaging speed of OCT enables the contrasting of dynamic structures (e.g., blood vessels) from static tissue. Thereby, two or more cross-sectional OCT tomograms (B-scans) are recorded at the same sample location at high speed. Any motion introduced by flow will cause changes of the amplitude and phase between the recorded OCT B-scans. As a result, vessel maps of tissue are generated and the technique is referred to as OCT angiography (OCTA), mainly because of the similarity of the images to conventional angiographic imaging. However, in contrast to most angiographic techniques, OCTA is completely non-invasive and does not rely on the administration of contrast agents. [PERSON] et al. [4] present an overview on the various algorithms used for OCTA and provide a detailed analysis on the question of whether the technique can be extended from a purely contrasting method to a more quantitative assessment of the underlying flow. By exploiting the polarization sensitive information carried by light, additional information on sample composition can be obtained. The corresponding technique is called polarization sensitive (PS-)OCT, which enables the contrasting of different tissue types based on their polarization sensitive properties. PS-OCT is not limited to the biomedical field, and can be very attractive for material sciences and non-destructive testing. Technological principles as well as the various applications of PS-OCT are summarized by [PERSON] [5]. An interesting and growing field of application for OCT is presented by [PERSON] et al. [6]. In their work, they provide an overview of OCT in dentistry and evaluate the performance of the technique for caries diagnosis and for the monitoring of composite restorations. OCT-based image-guided surgery is another growing field of application. Thus, fast imaging processing will be essential for translating this technology to clinical routine. In order to address this issue, [PERSON] et al. [7] present new segmentation approaches for localizing surgical needles that can be inserted into the eye ball for microsurgical procedures. The sintering process is an important step towards high quality fabrication of tooth prostheses. State of the art quality metrics of this process are either time-consuming or subjective. [PERSON] et al. [8] propose the use of OCT for a non-destructive investigation of temperature effects during this process. For specific applications, the image contrast provided by OCT might not be sufficient. To increase the visibility of structures, contrast agents can be employed that are usually highly scattering. A very elegant method is the use of iron oxide particles in combination with magnetic-induced motion of the particles. The corresponding technique is referred to as magnetomotive-OCT (MM-OCT). In their paper, [PERSON] et al. [9] present an improved method for MM-OCT and provide first imaging results in cells. Doppler OCT enables the non-invasive quantification of flow in tissue by measuring the axial velocity component.