  Measurements of the double-differential $\pi^{\pm}$ production cross-section
in the range of momentum $0.5 \GeVc \leq p \le 8.0 \GeVc$ and angle $0.025 \rad
\leq \theta \le 0.25 \rad$ in interactions of charged pions on beryllium,
carbon, aluminium, copper, tin, tantalum and lead are presented. These data
represent the first experimental campaign to systematically measure forward
pion hadroproduction. The data were taken with the large acceptance HARP
detector in the T9 beam line of the CERN PS. Incident particles, impinging on a
5% nuclear interaction length target, were identified by an elaborate system of
beam detectors. The tracking and identification of the produced particles was
performed using the forward spectrometer of the HARP detector. Results are
obtained for the double-differential cross-sections $ {{\mathrm{d}^2
\sigma}}/{{\mathrm{d}p\mathrm{d}\Omega}} $ mainly at four incident pion beam
momenta (3 \GeVc, 5 \GeVc, 8 \GeVc and 12 \GeVc). The measurements are compared
with the GEANT4 and MARS Monte Carlo simulation
