Wireless communication growth has been on the rise, reaching newer industry segments such as the automotive and health industries. Each segment imposes different requirements upon the communications systems such as ultra-high bandwidth or low latency, for example. In this context applications such as machine-to-machine M2M communications, Internet of Things (IoT), multimedia and Augmented Reality (AR) are requiring communications systems that can perform dynamic resource utilization. Network slicing will play a pivotal role in addressing varied use cases by enabling dedicated virtualized network slices for each use case. Network slicing is about transforming a PLMN from a single network to a network where logical partitions are created, with appropriate network isolation, resources, optimized topology and specific configuration to serve various service requirements. Slice Manager creates multiple dedicated virtual networks using a common physical infrastructure. Each virtual network slice is composed of independent logical network functions serving a specific use case. Each network slice will be optimized to provide the required resources and quality of service (QoS) with regards to latency, throughput, capacity, coverage and so on. This document discusses the specifications and Release A implementation of the 5GENESIS Slice Manager. The Slice Manager developed in the frame of 5GENESIS is responsible for communicating southbound with the management and control elements of the infrastructure and northbound with the 5GENESIS Coordination Layer components. Although 5GENESIS Slice Manager is mostly aiming in supporting the experimental features of the 5GENESIS facilities, it will also in the next releases implement additional features related to the network slice provisioning and service deployment. The Slice Manager is exploiting enabling technologies available at the infrastructure layer in order to be able to satisfy the aforementioned requirements. This document presents the detailed architecture of the Slice Manager and the functional Use Cases that it supports and concern the lifecycle management of the network slices. It discusses the overall architecture and provides information on each functional component of the architecture. The architecture of the Slice Manager is exploiting to design principles namely a) microservices and b) modularity. In this sense, the design allows implementation of additional interfaces or features without deteriorating system operation. The document provides detailed information on the available API for the northbound interfaces and implements interfaces with well-known MANO and NMS solutions.