I applied to Booking after a referral from a data scientist. Firstly there was a very easy phone interview with a recruiter, where I basically went over my background. Afterwards I had I phone interview with two members of the data science team, where I had to present a business case I worked on in the past, and solve 2 business cases with them. The last part of the process consists of half a day of interviews. The most important part of this are two interviews that follow a similar pattern as the phone interview. The difficult part of the interview is that you can be asked a very wide variety of problems that a data scientist can encounter. If you've already worked on something similar, then you will probably be fine, if not, the data scientists will give you hints but of course this will count against you and you also have too much time in your hands. Also, they are looking for people that will ace most of the business cases (3/4).