All students are obliged to have sufficient medical insurance before they come to the Netherlands. This means that medical expenses should be covered up until cost price in the Netherlands. In addition to coverage for medical expenses, TU Delft requires you to have insurance for personal liability, also called third party insurance for private individuals. In general your insurance needs to start on the day you leave your home country.
Which health insurance you need depends on several things; where you come from (EU/ Non-EU), how long you are going to stay, if you are getting a part time job or paid internship. Please note that the cost price of medical expenses in the Netherlands may be much higher than what you are used to.