The Muon to Central Trigger Processor Interface (MUCTPI) is part of the ATLAS Level-1 trigger system and connects the output of muon trigger system to the Central Trigger Processor (CTP). At every bunch crossing (BC), the MUCTPI receives information on muon candidates from each of the 208 muon trigger sectors and calculates the total multiplicity for each of six transverse momentum (pT) thresholds. This multiplicity value is then sent to the CTP, where it is used together with the input from the Calorimeter trigger to make the final Level-1 Accept (L1A) decision. In addition the MUCTPI provides summary information to the Level-2 trigger and to the data acquisition (DAQ) system for events selected at Level-1. This information is used to define the regions of interest (RoIs) that drive the Level-2 muontrigger processing. The MUCTPI system consists of a 9U VME chassis with a dedicated active backplane and 18 custom designed modules. The design of the modules is based on state-of-the-art FPGA devices and special attention was paid to low-latency in the data transmission and processing. We present the design and implementation of the final version of the MUCTPI. A partially populated MUCTPI system is already installed in the ATLAS experiment and is being used regularly for commissioning tests and combined cosmic ray data taking runs. I. ATLAS FIRST LEVEL MUON TRIGGER The ATLAS Level-1 trigger [1] uses information on clusters and global energy in the calorimeters and multiplicities from tracks found in the dedicated fast muon trigger detectors in order to reduce the event rate to 100 kHz with an overall latency of less than 2.5 μs. The muon trigger detectors are resistive plate chambers (RPC) in the barrel region and thin-gap chambers (TGC) in the end-cap and forward regions of ATLAS. Coincidences of hits in different detector layers are used to identify muon candidates. The muon trigger electronics also determines the transverse-momentum (pT) of the muon candidates and classifies them according to six programmable pT thresholds. The muon trigger detectors are divided into sectors, 64 for the barrel, 96 for the end-cap and 48 for the forward region. Each sector can identify up to two muon candidates. The trigger sector logic modules send information about the position and pT threshold of the muon candidates to the MUCTPI at the bunch crossing (BC) rate of 40.08 MHz. An overview of the muon trigger system is shown in Figure 1 below.