The aim of this thesis is to analyse and compare the contributions of business or startup accelerators. The contributions are compared from the point of view of the managers of accelerators, startups and investors. Business accelerators help new firms accelerate their growth. The aid can be provided in a material form through financial capital and services or in an educational form through workshops and mentoring. The first chapter deals with the definition of the accelerator, its history and differences with incubators. The second chapter engages in the causes of the phenomenon of startups and accelerators, the model of lean startup, the impacts of accelerators on the economy and the current situation of accelerators in the world. The third chapters analyses the oldest and the most famous accelerator in the world Y Combinator. The last part evaluates the contributions of accelerators based on the inquiry among experts.