Without the extinction event 66 million years ago, it is very unlikely that humans would exist today.  Our habitat is the world as it is today, and without a massive wipe out of dinosaur species, there would be no space available for mammals to evolve and expand into.  The fossil record certainly indicates that evolution of mammals happened in the wake of the dinosaur extinction event around 66 million years ago.
assistant
I’m not 100% sure, but here are some examples of some important things that would likely be different:

-   Dinosaurs were extremely successful and prolific in the Upper Cretaceous, some reaching a length of 30 meters (100 ft).  They were the dominant animals in Mesozoic life, and there would likely still be plenty of reptiles in many modern places, like the South and Central American rainforest.
-   Dinosaurs existed for 160 million years after the Cretaceous-Paleogene extinction event.  This equates to an additional 40 billion years of dinosaur life.  The lives of dinosaurs in the Jurassic, Cretaceous, and later periods would have been drastically different.  For example, there would likely have been very different polar environments in the presence of dinosaurs, and the potential for dinosaur extinction events like asteroid impacts would have been much higher, or dinosaur life could have simply died out naturally over time.
-   Other fauna and flora would also be different, such as megafauna like mammoths and mastodons.
-   A large number of different bird and mammal species, animals that humans are closely related to, would likely not exist today.  Humans themselves would more likely continue to evolve as a species, or possibly go extinct due to a disruption of their ecosystem.
-   Dinosaurs and large reptiles would greatly alter the modern oceans as predators and prey, and the ecological calamities brought about by their extinction likely resulted in the drastic change of global temperature patterns and significant overturn of the Earth's atmosphere over time. As this process occurred, humans would have developed increasingly complex civilizations of their own.
-   The dinosaurs did not evolve into birds in the absence of an extinction event.  Instead, birds would be a type of highly varied dinosaur, much the same way the modern birds are from multiple types of dinosaurs.  Birds would likely have originated from different dinosaur types than the avian dinosaurs that diverged from dinosaurs.  The winged reptiles and saurians found in modern times, referred to as pterosaurs or pterodactyls, would not have died out and would likely still be around today, in significantly larger numbers.  Human society would have been drastically altered by their presence.
-   Cetaceans, the modern order of life that includes whales and dolphins, would not have evolved.  A new order would likely have developed from the terrestrial dinosaurs, including the
assistant
What would have happened if dinosaurs were never wiped out?
user