The aim of this presentation is to introduce the participant into Time-of-Flight (ToF) Positron Emission Tomography and to give an overview on fundamental principles, state of the technology, fundamental limits, benefits and new approaches enabled by ToF-PET. Special emphasis will be put on the potential of ToF-PET for simultaneous MR/PET imaging. The course “Time-of-Flight: Do We Need It in PET/MRI?” will comprise 4 parts: Principles and technology of ToF-PET, General Benefits of ToF-PET, New Approaches Enabled by Time-of-Flight detection, and Future Technology Development and Fundamental Limits.Time-of-Flight PET was first studied and implemented in the early 1980s. However, these systems were experimental and were limited by scintillators, photo detectors and electronic components available at this time. After important technological progress was achieved on these key components, ToF-PET was readdressed in the 21st century and found its way into commercial products in 2006. It has become now stat-of-the art in nuclear imaging. Newly released commercial, clinical PET scanners have timing resolutions of about 400 picoseconds, which is rather close to the timing resolutions of 450 to 750 picoseconds achieved with the first experimental systems based on CsF and BaF2 scintillators. Compared with other scintillators, CsF and BaF2 have a poor intrinsic gamma ray detection efficiency (also called stopping power) and low scintillation light yield, leading to low spatial resolution of the PET systems and also low system sensitivity. Only the development of new, bright, fast scintillation crystals with high stopping power made ToF-PET feasible in clinical systems. With the starting interest in simultaneous MR/PET imaging, the photo multiplier tube had to be replaced by photo detectors that where insensitive to the strong electromagnetic fields of MR scanners. This propelled the development of silicon-based photodetectors and made Silicon Photomultiplier (SiPM) widely available. Apart from their compactness and MR compatibility, they also have excellent intrinsic timing resolution, which makes them a promising photodetector for ToF PET systems. In this part of the presentation, general concepts of ToF-PET and most important characteristics of the key components scintillator and photo detector will be discussed. ToF-PET is mainly characterized by an improved trade-off between image contrast and image noise. But it has also been demonstrated to enable faster convergence of iterative image reconstruction algorithms, to lead to better lesion detectability, better image homogeneity and making iterative reconstruction more robust in the presence of inconsistent data. As a consequence, ToF- PET allows for more challenging applications, e.g imaging with shorter examination and/or frame times, imaging with lower counts caused by low uptake, low injected dose or non-standard radioisotopes and imaging of larger patients. The underlying property that is responsible for the mentioned advantages is the effect that improving ToF resolution in PET scanner increases their effective sensitivity. This correlation between timing resolution and image SNR is often referred to as Time-of-Flight sensitivity gain will be introduced by means of examples from real patient examinations and simulations as shown in figure 1. Apart from the sensitivity gain achieved at constant acquired counts, Time-of-Flight measurements provide every detected temporal coincident event with an additional scalar value that gives the arrival time difference the both detected gamma photons. This additional parameter allows for new imaging application as simultaneous acquisition of events emitted by the object or emitted from a supplementary transmission source for PET attenuation correction, which is of special interest for highly precise simultaneous MR/PET imaging. Also it was shown, that bare emission data with timing information allow joint estimation of activity and attenuation images up to a global scale factor, that attenuation artefacts due to motion or MR image segmentation are reduced by ToF reconstruction and that Time-of-Flight also enables limited angle Positron Emission Tomography. During the last decade, huge progress was made in the development of PET detector technology, especially of silicon-based photodetectors. At the end of the course, a short outlook on expected future developments and also fundamental limits is given.