Supernova remnants (SNRs) are believed to be the particle accelerators for Galactic cosmic rays (CR) up to the energy of the CR knee at ~10^15 eV. Recent observations from space-based GeV observatories and ground-based TeV observatories indeed reveal GeV and TeV emission from middle aged SNRs, such as W44, IC 443, W28 and W51C. The characteristic pion-decay signature identified in the IC443 and W44 spectra provides possible direct evidence for CR acceleration in SNRs. All the above remnants show signatures of molecular interaction and the gamma ray emission regions are roughly spatially coincident with the molecular interaction regions. Based on observed molecular interaction and the widely accepted diffusive shock acceleration (DSA) mechanism for particle acceleration, two scenarios have been proposed to explain the observed gamma ray emission from these middle aged SNRs. In one, accelerated CR particles escape from the SNR and then illuminate nearby molecular clumps, producing gamma ray emission, while the other involves direct interaction between the SNR and molecular clumps. I will present a direct interaction model for radiative SNRs in addition to a time dependent DSA solution for particle acceleration in SNRs to explain the observed GeV and TeV emission. I will also compare the different models and discuss their requirements for the surrounding environment and limitations in application.