What distinguishes a groundbreaking idea from a mediocre one? This is often a challenging distinction to make. Take the example of vaccination: collecting pus from a cowpox-infected individual and injecting it into an eight-year-old boy may seem utterly reckless. Yet, 18th-century physician Edward Jenner’s daring action ultimately led to the eradication of smallpox, a disease that plagued humanity.

With the benefit of hindsight, we recognize that Jenner’s innovation was monumental. This principle of vaccination continues to save millions of lives today. As we progress through the 21st century, we feel it’s essential to reflect on and celebrate transformative ideas from the past 25 years that are reshaping our perspectives, actions, and understanding of the world around us.

Compiling our list of the 21 most impactful ideas of the 21st century involved rigorous discussions among our editorial team. One of our initial challenges was determining if the first quarter of this century would conclude at the beginning or end of 2025. For clarity, we opted for the latter. We navigated debates on various ideas, dedicating particular attention to concepts like the microbiome—establishing it as a legitimate 21st-century notion—and scrutinizing the role of social media, which after much discussion, we deemed largely negative. Ultimately, we recognize that the quality of ideas is subjective.

We developed a robust set of criteria for our selection. To qualify for this list, a concept must already demonstrate a significant impact on our self-understanding, health, or broader universe. Additionally, it should be grounded in scientific discovery, with a strong idea underpinning it. Lastly, the development must have occurred within the last 25 years.

While the last criterion may appear straightforward, we encountered numerous proposals that remain unrealized. The discovery of gravitational waves in the 21st century opened new cosmic vistas, but their prediction dates back a century to Albert Einstein. Similarly, ideas like weight loss medications, personalized medicine, and mRNA vaccines show promise, but their full potential has yet to be achieved—perhaps these will make the list in 2050.

During our selection process, we couldn’t disregard ideas that initially seemed appealing but faltered. Therefore, we also crafted a list of the five most disappointing ideas of the century thus far. The line between success and failure can sometimes blur, leading to controversial choices in our best ideas list. For instance, while many would advocate for the removal of smartphones, we ultimately view them as largely beneficial. Likewise, the ambitious global warming target of 1.5°C can be seen as a failure, especially as new reports indicate that average global temperatures have surpassed this benchmark for the first time. Nonetheless, we argue that striving to reduce the threshold from 2°C remains one of the century’s monumental ideas, setting a standard for global climate ambition.

Advancing away from fossil fuels is undoubtedly crucial, and prominently featured in this effort is Elon Musk. In 2016, before Musk ventured into social media and politics, his company Tesla launched its first Gigafactory in Nevada, marking a pivotal moment in the transition to renewable energy by utilizing economies of scale to transform transportation and energy systems. Conversely, other approaches to fighting climate change, such as alternative fuels and carbon offsets, appear more harmful than beneficial.

One significant takeaway from our selection process is that revolutionary ideas often arise by chance. For many, a working outlet can be the catalyst for a few minutes of smartphone scrolling during a lengthy commute. However, for two physicists in 2005, their discovery altered the global decarbonization strategy. This breakthrough also unveiled the foundations of our complex thought processes, illustrating that brain regions don’t operate in isolation but are interwoven into a robust network. This understanding has revolutionized our approach to diagnosing and treating neurological issues.

Looking back over the past quarter-century, it’s evident that the world has transformed considerably. We successfully dodged the Millennium Bug, the human genome’s first draft was completed, and the International Space Station welcomed its first crew. Concepts like “Denisovans” and “microbiomes” were unknown to us. In our pages, we celebrated innovations like wireless communication and marveled at miniaturized computer chips driving these technologies. “At its core is a device known as a Bluetooth chip,” we stated, positing it as the next big thing—a prediction that, in hindsight, was flawed, since truly transformative technologies extend beyond mere convenience.

This experience highlights the folly of predictions, as they can often be overlooked in the rush for the next trending innovation. Thus, rather than striving to foresee the future, we ought to invest time in contemplating the past. The advancements we’ve witnessed in health, technology, and environmental conservation suggest that this century has made the world a better place. Let’s hope, without necessarily predicting, that this momentum continues into the future.