An investigation of 356 antiproton annihilations in nuclear emulsion is reported. The results are generally analysed to yield information on primary processes, assumed to be annihilations with single protons or neutrons into a number of pions with or without a K-meson pair. The average pion multiplicity is 4.68±0.12 for annihilations at rest, and 5.11±0.12 for annihilations in flight (the antiproton energy being on the average 166 MeV). This increase of 0.4±0.2 pions is to be compared with an expected average increase of 0.1 pion. The pion energy distributions are given with mean values of 391±10 MeV total energy (at rest) and 390±9 MeV (in flight). Emission of K-meson pairs is found to take place in 3±2% of the annihilation reactions. The average number of pions emitted together with K-mesons is 2.2±0.5. The correction for K-meson pairs unobservable in usual emulsion experiments has been computed. The result that about 40% of the K-meson pairs are undetectable, means that the correction for detection efficiency is larger than previously assumed. A recalculation of all published data using emulsion detectors results in a world average of 5.0±1.1% of the annihilations with K-meson pairs for annihilations at rest and in flight (antiproton energy below 250 MeV). The annihilation probability at rest with neutrons is found to be less than that with protons on the basis of a determination of the number of stars with an odd number of charged pions relative to that with an even number. The ratio odd/even is 0.73±0.09. The reabsorption probability of the pions in complex emulsion nuclei is determined. It is found that 18% of the pions are absorbed on the average (at rest) and 32% (in flight). The difference is understood as a result of the movement of the centre-of-momentum and the penetration of the antiprotons into the nuclei for the cases produced in flight. Comparison of our experimental results with the predictions of various theories is carried out. For the pion production, however, no decision in favour of a particular theoretical model can be made. As regards the production of K-mesons, all theories fail to predict the experimentally found production frequency. The world data on antiproton annihilations are collected, reanalysed and discussed.