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We report on the Stark-deceleration and electrostatic trapping of metastable NH molecules. Furthermore the progress towards higher densities of cold neutral molecules by accumulation of multiple Stark-decelerated packets of NH molecules in a magnetic trap will be presented. NH molecules in the long-lived metastable $a1\Delta (v=0,J=2)$ state are ideally suited for Stark deceleration experiments because of their relatively large Stark shift and low mass. The metastable molecules $(\tau >2.7s)$ are produced in a supersonic expansion with a velocity of $\sim 450$ m/s, and are decelerated to a standstill by a 108-stage decelerator. Subsequently the metastable molecules are trapped electrostatically, with a temperature of about $50-100$ mK, a density of $\sim 106$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-3</mrow>}$ and a $1/e$ trapping lifetime of $1.4$ s. Following the deceleration and trapping, the metastable NH molecules are detected by the excitation of a spin-forbidden transition, resulting in spontaneous decay to the electronic ground state ($X3\Sigma -$). The electronic ground state has a negligible Stark shift, but can be trapped magnetically. The first experiments on the accumulation of ground state NH molecules in a magnetic trap will be presented.\ [1] S.~Hoekstra \emph{et al.}, Electrostatic trapping of metastable NH molecules, Phys.~Rev.~A. \textbf{76} 063408 (2007)