CitySDK (ToolKit for City Service Development) started as a pan-European project that aimed at creating open interoperable interfaces between cities in 8 major cities in Europe. The toolkit now consists of open and interoperable digital service interfaces as well as other resources that enable more efficient utilization of the expertise and know-how of developer communities in the city service development. CitySDK first undertook three domains: participation, mobility and tourism and is now preparing to extend to new cities and domains. The webinar uses the experiences from Helsinki and Istanbul to showcase the possibilities that harmonization of Smart City APIs can offer, no matter how advanced a city is in regards to open data.