癌症有超過一百個種類，一般可依據癌細胞原發部位來分類，例如肺癌、乳癌等；另外能根據不同組織或細胞做以下分類：
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Cancers generally crop up from an assemblage of mutations and epimutations that confer a selective edge leading to clonal expansion (see Industry defects in development to most cancers). Mutations, however, is probably not as frequent in cancers as epigenetic alterations.
註解：癌細胞將於所到之處繼續失控的繁殖。舉例：若肺癌細胞轉移到肝臟，則肝臟將出現不斷增生的肺部細胞（不具肺功能），且此細胞具有入侵正常肝臟細胞的能力，導致肝功能不斷降低；反之，不具入侵能力但失控繁殖的細胞，稱為良性腫瘤。
另一方面，現在有些方法則是想利用身體本身的免疫系統產生非專一性的免疫反應來對抗癌細胞，例如卡介苗膀胱內灌注免疫療法（英语：intravesical BCG immunotherapy）將卡介苗灌注進入膀胱內，以引發免疫反應對抗表淺性膀胱癌（英语：superficial bladder most cancers）。此外使用干擾素和介白質素，均可引起免疫反應。而使用疫苗所產生的免疫反應來對抗腫瘤的研究也正在進行當中，尤其是針對惡性黑色素瘤與腎細胞腫瘤（英语：renal cell carcinoma））。
活體組織切片後取下的組織，經染色後由病理醫師判讀，此圖所示為惡性黑色素瘤之切片 病患可能經由許多症狀或檢驗結果被懷疑罹患癌症，但診斷惡性腫瘤最確定的方式就是經由病理醫師對活體組織切片或經手術取得的組織做病理檢查來證實癌細胞的存在。許多活體組織切片，例如皮膚、乳房或肝臟等，在醫師的診療室內即可取得。但是其他器官的切片，則需要在麻醉情形下在手術室進行。
症状性癌症的早期诊断在所有环境中并且对大多数癌症都意义重大。癌症规划应旨在减少诊断、治疗和支持性护理方面的延误和障碍。
局部症狀：異常腫塊或腫脹（腫瘤），出血，疼痛和潰瘍。因為腫瘤壓迫周遭組織所引起的症狀，如黃疸。
In the 1970s, a relatively common alternate most cancers treatment inside the US was a specialized kind of converse therapy, based on the idea that most cancers was caused by a bad Mindset.[244] People with a "most cancers personality"—frustrated, repressed, self-loathing and fearful to precise their feelings—have been thought to get manifested cancer via subconscious desire. Some psychotherapists claimed that therapy to alter the client's outlook on lifetime would get rid of the cancer.[244] Amongst other consequences, this perception allowed Modern society accountable the victim for having brought about the cancer (by "seeking" it) or obtaining prevented its treatment (by not getting a sufficiently satisfied, fearless and loving particular person).
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物理致癌物，例如紫外线和电离辐射；而异物刺激、创伤、慢性炎症等会促进恶变进程。
Folks who report a 癌症 higher Standard of living have a tendency to survive longer.[209] Those with decrease quality of life could possibly be affected by depression and various problems and/or disorder progression that equally impairs high quality and quantity of life. Furthermore, patients with even worse prognoses might be frustrated or report poorer quality of life as they perceive that their affliction is likely for being deadly.[citation wanted]
Multiple nationwide health-related pointers suggest early palliative care for patients whose cancer has generated distressing indications or who have to have support coping with their health issues.
微量營養素缺乏被認為與癌症有關，但是進行隨機臨床實驗需要眾多人數參與，需時多年才能完成，因此花費也極為昂貴和複雜，所以很少實施。而通常只能以單劑量來做實驗，缺乏其他濃度比較，使得難以評估實際臨床所需的量也是一大問題。